Bootlegging is also the term used for the creation and sale of copies of illegal recordings - specifically live recordings, outtakes and other material the creator hasn't authorised for release. These recordings, reasonably enough, are called bootlegs.
Bootlegging is distinct from piracy, which is the creation of unauthorised copies of released recordings. Many people find bootlegging more morally acceptable than piracy because bootlegs tend only to be bought by people who already own all the legitimately released recordings of the artist, while pirate copies take revenue away from the copyright holder.
Some famous bootlegs include:
Smile - the Beach Boys
Great White Wonder - Bob Dylan
Get Back - The Beatles
But almost every artist with a substantial following, from Eminem to the Beautiful South will have a plethora of poorly recorded live CDs available at your local record fair...