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, criticism and news reporting. Whether the use of copyrighted material qualifies as "fair use" depends on the purpose and character of the use, the nature and character of the copyrighted work, the amount and substantiality of the portion used, and whether the use adversely affects on the value and commercial potential of the copyrighted work. The "Fair Use Doctrine" has been continually molded by more than two centuries of legal precedent  & . Consequently cases are judged not only by the optics of each individual case but also by years and years of legal precedent.
Under the (albeit uncertain) protection of the "Fair Use Doctrine," artists of all varieties have continued to push the boundaries of the exclusive rights given to copyright holders. One popular example is remixing music. Musicians take elements from the finished product of other musicians and combine them (with original elements or other "found" materials) so that a new song is created. This practice is done primarily in hip-hop and is called sampling. 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. A good example of this is DJ Danger Mouse's Grey Album, a remix of The Beatles White Album and Jay-Z's Black Album, which was not released due to copyright infringement.
Interestingly, when this page is in edit mode towards the bottom of the page, there is a large bold statement that states, "DO NOT SUBMIT COPYRIGHTED WORK WITHOUT PERMISSION!" Even when amassing information for a purpose such as this one, the explanation and cataloging of information, there is a hyper-awareness of the need to protect the exclusive rights of copyright holders.
On the other hand another site, Youtube, which is also user-driven and fundamentally concerned with the explanation of and cataloging of information, is notorious for it's gross underestimation of the "Fair Use Doctrine." Unlike many other 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."