SOPA is a piece of legislation that, if passed, could essentially shut down most of the internet. It doesn't just affect fan works that are in legal gray areas, it affects far, far more than that.

Other people can explain why SOPA is evil much better than I can, so try the following links:

http://americancensorship.org/

http://fightforthefuture.org/pipa/

https://www.eff.org/deeplinks/2011/10/sopa-hollywood-finally-gets-chance-break-internet

http://arstechnica.com/tech-policy/news/2011/10/house-takes-senates-bad-internet-censorship-bill-makes-it-worse.ars