DJ stands for disc jockey, and means anyone who mixes tracks to provide music to others. There are a few different definitions of what a dj is/does:

Club DJEdit

A club DJ is one who mixes tracks by beatmatching.

TurntablistEdit

A turntablist is one who uses the turntable as an instrument. See turntablism for more details.

Mobile DJEdit

A mobile DJ is one who provides their own equipment set up and is hired out for small parties; mixing is practiced less often in this profession.

Radio DJEdit

A radio DJ is one who selects tracks for play on air, over a broadcasting medium like FM radio or internet radio. Tracks may or may not be mixed, and chat is often added by the DJ to add interest to the show.