It's basically the study of how and why we make language choices. People use certain forms to show that they are members of a social group, for instance. Looking at the ways we do this is stylistics. Here's the Wikipedia definition:
Stylistics relates to the study of style used in literary, and verbal language and the effect the writer/speaker wishes to communicate to the reader/hearer. It attempts to establish principles capable of explaining the particular choices made by individuals and social groups in their use of language, such as, socialization, the production and reception of meaning, the question of perspective, literary criticism, and critical discourse analysis.
http://en.wikipedia.org/wiki/Stylistics_(linguistics)