Where's Webster when you need him?
A French term meaning "front guard", or "advance guard." It should be hyphenated; avant-garde. The unhyphenated form is becoming more and more popular, but beware pedantic professors -- they'll want it written the traditional way. Pronunciation is highly flexible, even in Webster's book; avan gard, avon gard, avant gard, and avan(t) gaud are all acceptable.
This was originally applied to advance military forces. It came to the English language in the 1600s, and somewhere around 1900 it began to be used in the non-military sense. These days it's used to mean a group that uses original, unique, unorthodox, and/or experimental ideas or techniques. Avant-garde generally implies that the group is ahead of its time. It is most often applied to the arts.