Wikipedia will go offline in opposition to the Stop Online Piracy Act (SOPA), a bill that would allow the U.S. government to cut off funding sources to foreign websites accused of piracy by rights holders. PIPA, the Protect IP Act in the Senate, and SOPA, the Stop Online Piracy Act, have been presented as a way to protect movie studios, record labels and others. Supporters range from the Country Music Association to the U.S. Chamber of Commerce.
Websites found to offer pirated content, along with the services that they use, could be hidden from US internet users by being delisted on search engines and potentially on DNS servers themselves.
The Internet giants say the bills could require your Internet provider to block websites that are involved in digital file sharing. And search engines such as Google, Yahoo and Bing could be stopped from linking to them -- antithetical, they say, to the ideal of an open Internet.
The whole explanation you can find on Wikipedia, or just click here.