It is not hard to draw an image of mask in the mind upon hearing the word, especially in this day and age when masks are not only used in theatres but also sold as a popular commodities in stores for such occasions as Halloween. How many little boys in hockey masks have we seen on the streets begging for candy, or Shreks or Nemos? Even the non-theater-goer can recognize what a mask is thanks to the capitalist economy of the United States. However, masks are not important merely on a commercial level. The concept has also succeeded in the infiltration of the idiomatic lexicon. "Today we talk of tearing the mask off someone's face or unmasking him, meaning that we have removed his disguise as an honest man and exposed him for what he really is.”  This use of the word "mask" in a figural way is not far off from the original meaning, which, very generally, is considered to be anything that conceals or disguises the face.
In fact, with that definition, cultures around the world have produced a great number of masks. If we attempt to classify them, it can be catalogued very broadly into four categories. Two, I've already mentioned, namely, theatrical masks and figural masks. It might be relevant to note here that mask, in terms of figurative language, is used in a negative way. It is seen to be the primary object that obscures the truth and supports the false intentions of the wearer. Language is a social tool that shapes the way people of the age think and perceive of things or vice versa. Language is also the result of the way people think and perceive of things. In a time period in which reason is the prevailing value, people do not appreciate being duped or tricked. They do not like having their reason be subjugated to falsehood. Hence, the unmasking of somebody becomes a positive term where wearing a mask becomes something of shady business.
The third category is utilitarian. Workers wear gas masks in a toxic areas. Dentists wear surgical masks. Children wear wool masks in the winter time to protect their faces from the cold. A person can go to Walgreen's and head to the cosmetic section to find a variety of beauty masks: aloe, mint, tea tree etc. Even the foundations and lipstick women wear on a day to day basis can be considered a type of mask. In fact, it can be considered a soft mask, which falls also under one or two of the other categories, a point I will flesh out in more detail later. Needless to say, these masks, just by being useful in the practical sense, have lost their mystery and power we have learnt to associate with masks. But aura or no aura, one undeniable fact is their relation to folk art which first produced the mask as we know it today.
Masks produced early on fall under the fourth category, the spiritual.  In Africa, masks were used primarily to worship the ancestors. They came in a variety of forms, though the prevalent themes were animals and/or replications of the human face and body. I say body, because masks often were big enough to cover several people not to mention one, in which case the mask would not just be a manifestation of a face but the entire body.  One of the more striking example of masks used for ritualistic purposes were the secret society masks. These masks were meant to represent an entire society and people outside of it were forbidden to look upon it. In fact, it was destroyed after the ceremony.  This destruction is important in that it shows that the mask is insignificant outside of the person wearing it and who gives it movement. The mask and the ceremony were tied together and the power of each was greatly diminished in the absence of the other.
Though in African mask history, the activity bestowed significance on to the mask, there are instances in history where masks were valued for their aesthetic purposes. Although in some sense theatrical masks were also used ritualistically, they were not destroyed after the production, establishing the masks as independent artistic objects. Japanese masks were marked for their high stylization. "Here we find the purest translation of life into art- a complete 'artificiality' -in a strict, absolute and utterly unnatural stylization."  Perhaps a good example would be masks that served the No theatre. The demons were literary demons, symbols of human passion and their masks were created to reflect this symbolism. Likewise, the masks of other characters carried their essential distinguishing traits so that a viewer could tell, should the masks suddenly come alive on its own, what sort of persons they would be and what messages they would convey. The masks represented the stock characters of old man, woman, warrior and demon, whether or not they were used in the actual performance. 
Though their artistic independence has been stressed, that is not to say that their function stops at the visual. Rather, they were made for the purpose of the theater to enhance the character that which they portray. In this way the mask fulfills what Mcluhan terms an extension.  In the case of the theater, masks serve as extensions of the personality. The mask in the theatrical sense, according to the OED, is "an image of the face worn by an actor; a hollow figure of a human head intended both to identify the character represented and to amplify the voice."  In other words, the audience would not believe the actor was an old man if he did not look like an old man. In theater, therefore, the function of the mask is two fold. First, it helps the actor to get into his role. Second, it helps the audience suspend disbelief. The mask supplements, therefore, both the object and the interpreter.
Physically, the mask adds layers of skin on to the person whether it be a hard mask or soft mask. Hard masks refer to the facial covering made of wood, metal, clay or plastic. It must be made to fit the face of the wearer or not be worn at all. Soft masks, on the other hand, include powder, lipstick, eye liner, rouge, black face, white face, war paints and so on. They do not need to be pre-fitted as they are easily applicable and just as easily erasable, molding to the face perfectly.
The effect of soft mask is much more personal than a hard one. Thinking in terms of Mcluhan's principle of medium being extensions of the body, I might argue that the soft masks are closer to the original body than hard masks are, which physically extend the face much more than makeup does. The consequence is that the audience may be less "numb" to a performance in soft mask than perhaps they would be to a performance in hard mask. "All imitation should be given a hint of dissimilation for if an imitation is carried to the extreme, it offends against reality and ceases to give an impression of similarity."  In other words, if the imitation is too close to what it is imitating, it no longer becomes an impression but approaching an actuality. With hard mask, the audience is desensitized to what it is representing and may view upon it with detachment. However, with soft mask as the operating medium, the audience is forced to relate and view the performance with chagrin, guilt, affirmation or any number of emotions.
Because of this important difference, satire is often performed in soft mask. "Every culture is identified by a central core of values that find their social expression in idealized or normative behavior and attitudes. If the norms are in any way threatened, the comic or satiric minds of the culture expose the extent to which they have been twisted or ignored, invariably relying on a widespread acceptance of the norms and the ability of their audiences to recall them at the very moment they are witness to the comic deviations."  Dramatic satires necessitate the audience's ability to recognize the story as having likeness to reality. People laugh in the case of dramatic satire because they identify the aberration and think on the disparities in relation to actuality. The function of dramatic satire, which is to comment on society, depends heavily on this sort of audience recognition. It is no surprise, then, that these sorts of performances use the soft mask in their costuming schemes. Soft masks bring the show and reality of the viewer that much closer together.
A striking example of this is the blackface used in the film Bamboozled. The opening line is "This is a satire."  In fact, Peerless' justification for creating the minstrel show in the first place is that the show is satirical in nature. The reaction the minstrel show gets is mixed. Initially it is seen with shock. Little by little, however, it gains popularity and the audience becomes involved with the show to the extent where they themselves put on blackface. I can't help but wonder if the effect would have been different if Manray had worn a hard mask instead. Would the audience have the same feelings of identification? Would they have been able to relate to the show as well?
Though enhancing understanding is a key element of the mask (i.e. in theatre), the other side of the coin is its obstruction. Masks create a separation not only between subjects between alienates the self from the self as well. It produces psychological effects in both the person wearing it and the person viewing it. Masks have the power to forge a new identity as a face conveniently placed outside of the original face which is the primary source of recognition. Artistotle in his Physiognomics and Plato in Greater Hippias both assert that intelligence and morality, respectively, are directly associated with the face.  The person's value can be accurately gauged by the way a person looks. So, by hiding the face a person is in essence separating the original being from the masked being. In fact, in the realm of figural masks there are idioms like "put on a new face" "poker face" "save face" and so on which create and/or recognize the different identities.
People do things they would never have done in their own faces under the new persona of a mask. They become somebody else that they may at any time throw away. Lynching and torture are carried out by people in hoods with circles cut out for eyes. Bandits rob under the anonymity afforded by the mask. Crimes are at an all-time high during Halloween. A normally quiet individual may become loud and debauch under the veil of the night. Because the face acts as a primary identification label, to cover, conceal and disguise it makes possible a number of acts that would usually have been out of character. Even something as mild as everyday makeup produces a change in the person, that is, a rise in confidence that is not felt in the bare face.
Beyond the self-centered perspective, furthermore, the mask separates subjects from each other becoming an important medium of misunderstanding. The mask, as mentioned before, needn't be one that is worn physically. In fact the OED expands the definition of mask to include both the material and the immaterial.  Stereotypes, therefore, fall under the category of masks in that they disguise the truth and project a preconceived and generic image. W.J.T. Mitchell calls a stereotype an invisible mask that is "painted or laminated directly on to the body of the living being and inscribed into the perceptual apparatus of a beholder."  However, Mitchell asserts, stereotypes are a necessary evil in that it is by stereotypes that people can even begin to organize their perceptions. In this way, masks are integrated into everyday life. However, it presents a problem in that it amputates, using Mcluhan's terms, the subject's soul, so to speak. The person is reduced to a couple of generic definitions and in some cases, the mask is never penetrated. The gravity of this sort of misunderstanding is reflected in the arts. For instance, in Paul Laurence Dunbar's poem "We Wear the Mask," the narrator laments how "we wear the mask that grins and lies/it hides our cheeks and shades our eyes/ this debt we pay to human guile;/ with torn and bleeding heart we smile/ and mouth with myriad subtleties."  A person does not have to go to a masquerade ball to be alienated from his fellow man; he has only to live.
The consequent alienation produced by masks does not always act as a push-away mechanism. Although it is true that that is their tendency, masks can also act to draw people in. Fanon, in "Algeria Unveiled," gives an account on how the veil acts to pique the white man's interest and spark his fantasies. "Hiding the face is also disguising a secret; it is also creating a world of mystery, of the hidden."  Fanon argues that it is the aggressive tendency of man to want to unmask that face and make it "available for adventure," which is precisely why the white colonizer worked so adamantly towards unveiling the Algerian women.
There are four broad ways of approaching the definition: masks as theatrical, figural, spiritual and/or utilitarian. Even within these categories, however, there two further types to be investigated: the hard mask and the soft mask. As far as differences go, the hard mask and the soft mask produces varying levels of intensity and psychological effects, which I've shown through the example of pantomime and Bamboozled. Furthermore, masks have frightening power over the human psychology by which they can effectively create a new identity. Masks are distinguishable from the face in that they are not the truth as Aristotle and Plato claim that the face is, but instead create a new image, material or immaterial. An example of an immaterial mask would be stereotypes. Though masks act as an extension of the body in that they add layers to the skin, they are complexes of reduction in that they amputate a person's soul. Masks become both a medium of understanding and misunderstanding.