The RIAA, MPAA and copyright holders describe P2P users as "pirates" - invoking images of swashbuckling pre-teens hauling up the Jolly Roger and stealing intellectual property in the dead of night. MPAA President Jack Valente (in the States) impress the idea that "copying is stealing" and that someone who burns MP3s is no different from those who slip a CD under their shirt at the local Tower Records.
But technically, file sharing is not theft.
A number of years ago, the U.S. Supreme Court dealt with a man named Dowling, who sold "pirated" Elvis Presley recordings, and was prosecuted for the Interstate Transportation of Stolen Property. The Supremes did not condone his actions, but did make it clear that it was not "theft" -- but technically "infringement" of the copyright of the Presley estate, and therefore copyright law, and not anti-theft statutes, had to be invoked.
So "copying" is not "stealing" but can be "infringing." That doesn't have the same sound bite quality as Valente's position.
Complicated matters further, copying is not always infringing. If the work is not copyrighted, if you have a license to make the copy, or if the work is in the public domain, you can copy at will. Also, not all "copies" are the same. Say you buy a CD and play it on your computer -- technically, you have already made a "copy" onto the PC in the process of playing it, but that's not an infringement.
Making an archive copy is okay too, as long as your retain the original. What about a transformative copy -- say, making an MP3 out of a CD? You can do that, so long as you retain the original work. If the original CD get scratched, damaged or lost, you can probably burn the MP3 back to a CD (sans the really "sucky" titles), but this is not entirely clear.
So the RIAA and MPAA's claims that all "copying" is "stealing" are much overhyped.