1. to make a copy of something you have so someone else can get one for free.
2. to deprive somebody of what he might have bought with your money, by getting for free a product of his you wouldn't spend good money on anyway.
1. "If you illegally copy music, it's stealing; it's exactly like picking my pocket!" -defenders of the RIAA
2. "File sharing steals food from my mouth!" -some washed-up heavy metal musician.