to a lower or outer limit or edge, as of social standing. (definition from answers.com)
A more recent buzz-word of the media, the act of marginalization can mean many different things. However, marginalization usually occurs when a value is imposed on certain characteristics of a genre, and consequently whatever does not meet those guidelines is viewed as worthless or marginal.
For instance, the "Great Masters" are viewed as being the "best" of the High Renaissance art movement. This is because they most accurately produced in their work the aesthetic values at the time. By creating a canon, we automatically marginalize all works not included.