A bootleg is an unofficial or unauthorized production – usually a remix or a mashup of another artist’s work.
Technically, any remix or mashup made without official legal permission from the artist whose work is sampled is a bootleg. Not all bootlegs will be labeled as such, though, because it’s pretty easy to figure out if a song is a legitimate release: If a song is available on music services like iTunes or Spotify, or otherwise available to buy, it’s almost certainly authorized. If a remix is only available as a free download, for example through an artist’s Soundcloud page or website, it’s probably not authorized. (This is because the original artist or their label could sue the remixer for damages if the remixer were trying to make money off of their work.) DJs big or small can still play bootlegs live – after all, mixing together other people’s work is the core of the DJ’s job description – so bootlegs will often make appearances in live settings as a special treat for the audience.
Some bootleg remixes are actually rejected remixes that had been commissioned by the original artist. Artists will frequently ask other producers to remix a song of theirs, to be released alongside the original mix on a single. When the commissioning artist is not happy with the remix, though, he or she can choose not to officially release it. Often, these rejected remixes will never see any release at all, but sometimes the remixer will release the song for free, just to get it out into the world. In other cases, the remixer may end up turning the remix into an original production of their own – as was the case with Above & Beyond‘s “Sticky Fingers.”