What is 501(c) Organization?

Legal Definition
A 501(c) organization, or simply a 501(c), is a tax-exempt nonprofit organization in the United States. Section 501(c) of the United States Internal Revenue Code (26 U.S.C. § 501) provides that 29 types of nonprofit organizations are exempt from some federal income taxes. Sections 503 through 505 set out the requirements for attaining such exemptions. Many states refer to Section 501(c) for definitions of organizations exempt from state taxation as well. 501(c) organizations can receive unlimited contributions from individuals, corporations, and unions.

The most common type of US tax-exempt nonprofit organization falls under category 501(c)(3), whereby a nonprofit organization is exempt from federal income tax if its activities have the following purposes: charitable, religious, educational, scientific, literary, testing for public safety, fostering amateur sports competition, or preventing cruelty to children or animals.
-- Wikipedia