A nonprofit organization is registered with the state government and the federal government's Internal Revenue Service under IRS Code, Section 501(c), for tax exempt status. Within this legal entity, the organization can generate income for self-preservation to address a specific mission. Members of a board and management oversee paid staff and volunteers in the administration of the nonprofit’s purpose.
Nonprofit organizations don't have the same reporting guidelines as corporations, but there are still some sources that cover nonprofits--especially because they are defined by the tax exempt status. Many of them are intended to help people make good donation choices, but they will also contain the information you need for a successful job hunt.
The resources in the company section will cover some nonprofits, and the industry sources will also cover industries related to the nonprofit sector. For example, you can find a nonprofit sector industry report and profiles for nonprofits in the Vault. You can also always start at a group's website if you have one already in mind, but the following specialized sources will provide a more complete picture.