What is Wikileaks?
Wikileaks is a self-described “not-for-profit media organization,” launched in 2006 for the purposes of disseminating original documents from anonymous sources and leakers. Its website says: “Wikileaks will accept restricted or censored material of political, ethical, diplomatic or historical significance. We do not accept rumor, opinion, other kinds of first hand accounts or material that is publicly available elsewhere.”
More detailed information about the history of the organization can be found on Wikipedia (with all the caveats that apply to a rapidly-changing Wiki topic). Wikipedia incidentally has nothing to do with Wikileaks — both share the word “Wiki” in the title, but they’re not affiliated.
An excellent Wikileaks FAQ resource on Jonathan Zittrain's blog