What happens, legally speaking, when a group of people get together and decide to perform some task without filing any legal paperwork or establishing any formal legal structure? Whether they know it or not, they have formed an unincorporated association. "Unincorporated association" means an unincorporated group of two or more persons joined by mutual consent for a common lawful purpose, whether organized for profit or not.
Now, if the lawful purpose they've joined together to accomplish includes earning a profit, their association is automatically a partnership or joint venture for tax and most other legal purposes. For example, if two people get together and decide to operate a food truck, they've formed a partnership, even if they file no paperwork.
But, if the purpose for the association is to benefit the public some way, and does not include earning a profit, the association's members have formed an unincorporated nonprofit association. People form nonprofit unincorporated associations all the time; often without being aware of it. For example, if you and several of your neighbors get together to help raise funds to keep your local library branch open, you've formed an unincorporated nonprofit association.
If an unincorporated association's purpose is charitable, educational, and/or scientific in nature, it can qualify as a Section 501(c)(3) organization (also called a public charity). Contributions to Section 501(c)(3)s are tax deductible. If an unincorporated charitable nonprofit has less than $5,000 in annual revenues, it may function as a 501(c)(3) without applying for IRS recognition of its status. However, as a practical matter, it may be difficult to obtain contributions without an IRS determination letter officially recognizing the nonprofit as a Section 5010(c)(3) organization.
An association with over $5,000 in revenue must apply for recognition from the IRS by filing IRS Form 1023. It is not necessary for an unincorporated association to convert to a nonprofit corporation to obtain IRS recognition of its Section 510(c)(3) status. However, the association must adopt written bylaws or a constitution, and include it with its IRS application. It's probably easier to form a nonprofit corporation than to adopt such bylaws or constitution.
The biggest drawback to the unincorporated nonprofit association, and the reason nonprofits often abandon this form in favor of a nonprofit corporation, is that it has no separate legal existence apart from its members. Because it is not respected as a separate legal entity, its members generally can be personally liable for its debts and liabilities. Some states, such as California, give some limited liability to nonprofit association members; but it's not as good as the protection obtainable from a nonprofit corporation. Moreover, unless your state law contains an "enabling statute" granting such rights entities, an unincorporated association cannot hold or receive property, or sign contracts, in its own name.
Because of these limitations, nonprofit unincorporated associations are usually used to accomplish limited short-term goals, such as raising funds for a library. Nonprofits with long-term missions should usually incorporate. For more on incorporating, see Nolo's article, Five Reasons to Incorporate Your Nonprofit.