Use the adjective pretentious as a way to criticize people who try to impress you as more important or knowledgeable than they really are.
A piece in Time Out New York railed against "pretentious gallerists and collectors who buy and sell graffiti-inspired art." These people, the writer means to suggest, think they know more about art than they really do. You might not be surprised to learn that it's related to the word pretend, and it fits the bill for someone who's only concerned with making an impressive appearance. If you don't want to be accused of being pretentious, just act naturally and don't put on airs!
