SOPA, or the Stop Online Piracy Act, was introduced in Congress last year and would give law enforcement agencies the power to censor the Internet, or to "combat online piracy and theft of intellectual property," depending on who you ask.
The bi-partisan bill, initially co-sponsored by 12 lawmakers, would allow the U.S. Justice Department to go after websites for "enabling or facilitating" copyright infringement, which means just about any site on the Internet could likely come under scrutiny. The full bill runs 78 pages. If you need help getting to sleep you can read the full thing here.
Google was probably one of the biggest sites to lend a voice to the strike (meaning they put a black square over their well-known logo on the Google.com search page).
Reddit also joined in the strike with a message stating, "SOPA and PIPA damage the Internet. Today we fight back." Unlike other sites like Google that simply gave a nod of support for the SOPA stikers (but still maintained functionality) Reddit actually shut down for a period of time.
Craigslist opened to a black screen telling the "corporate paymaster" to "keep those clammy hands off the Internet" with a link to more information about SOPA.
Wikipedia is also blackening its page for 24 hours and providing a nifty link to contact Congressional representatives.
In response, Representative Ben Quayle, one of the initial sponsors of the bill, says he's no longer supporting the legislation.