It's the process of copying the audio or video data from a media form (e.g. CD, DVD...) to a hard disk.
In simple words:
If you download a song from the net illegally, you say it's been copied or it's been ripped by sopmeone else before, who has done you the big favor of not having to spend your money and buying it from a shop.
Title: Lost Highway
Author: Bon Jovi
URL: Ripped by Saint_Nightmare
last night i downloaded Bon Jovi's new album off the interent, ripped by my friend who just bought the new CD