The fair usage copyright (legal) concept is an important one to be aware of - you may find yourself with a range of notices on your own channel, from notification that rights-holders can insert ads before your videos to removal of audio through to video deletion and even account deletion; you need to take care with how you handle copyright material.
Abuse of the system isn't just one way though, and Google (YouTube conglomerate parent) is starting to fight back against vexatious claims and profiteering by the record industry, film companies and other cynical profiteers...
