Typically it stands for Disc Jockey, you know, someone who can actually mix music. Now it's used by any and every person to download two mp3's and the demo version of FL Studio.
Person: Hey I heard you're a DJ!
DJ BJ: YEA BRO I GOT THIS ONE TRACK IT'S HOT
Person: How long you been doing that?
DJ BJ: WELL IT TOOK AN HOUR OR TWO I SPED UP THE PITCH
Person: No, I mean DJing...
DJ BJ: SHUT UP FUCK YOU IM A DJ