501(c) is a provision of the United States Internal Revenue Code (26 U.S.C. § 501(c)), listing twenty-eight types of non-profit organizations exempt from some Federal income taxes. Sections 503 through 505 list the requirements for attaining such exemptions. Many states reference Section 501(c) for definitions of organizations exempt from state taxation as well. - Wikipedia at: http://en.wikipedia.org/wiki/501%28c%29