IRS Publication 557 - Tax Exempt Status for Your Organization
"This publication discusses the rules and procedures for organizations that seek recognition of exemption from federal income tax under section 501(a) of the Internal Revenue Code (the Code). It explains the procedures you must follow to obtain an appropriate determination letter recognizing your organization's exemption, as well as certain other information that applies generally to all exempt organizations. To qualify for exemption under the Code, your organization must be organized for one or more of the purposes specifically designated in the Code. Organizations that are exempt under section 501(a) include those organizations described in section 501(c). Section 501(c) organizations are covered in this publication."