The "Fair Use Doctrine" refers to a portion of US Copyright Law, Section 107, which permits limited use of copyrighted materials without permission from the copyright holders for certain functions such as scholarship and parody. Whether the use of copyrighted material qualifies as "fair use" generally depends on the purpose of the use, the original copyrighted work, the amount of the copyrighted work used, and whether the use infringes on the commercial potential of the copyrighted work. The "Fair Use Doctrine" has been continually molded by more than two centuries of legal precedent .
Following the "Fair Use Doctrine," artists of all varieties of mediums have been challenging it in the past and today. One popular example is remixing music, because musicians just pick and choose songs and remix them to their own tastes, as primarily done in hip-hop. Usually, artists get away with making remixes and/or mash-ups, but some run into trouble with the original owners if the source is not in the public domain. Actually and interestingly enough, if you look towards the bottom of the page while editing, there is a large bold statement that states, "DO NOT SUBMIT COPYRIGHTED WORK WITHOUT PERMISSION!"
One site which is notorious for it's gross underestimation of the "Fair Use Doctrine", is Youtube. Unlike many video streaming sites, youtube's policy on fair use is an afterthought at best. Their copyright policy page encourages artists to upload entirely original video, so as to avoid any possible trouble with copyright. This is very obviously crippling creative artists. There is a small section on fair use, but it essentially exists if only to say "We are not responsible for your actions."