Organizations
The organizations listed here fall under one of two tax codes, 501(c)(3) or 501(c)(6).
501(c)(3)
This is the most common type of nonprofit. It includes organizations that are religious, educational, charitable, scientific, and literary; groups that test for public safety, that foster national or international amateur sports competition; or organizations engaged in the prevention of cruelty to children or animals.
This type of nonprofit applies for its status using IRS form 1023, and files annually form 990, 990EZ, or 990-PF. Contributions are usually tax-exempt.
501(c)(6)
These organizations are business leagues, chambers of commerce, real estate boards, etc. They seek to improve business conditions. They apply using IRS form 1024 and file annually the 990 or 990EZ.
See How the IRS Classifies Nonprofit Organizations on About.com for more information.