Friends, volunteers and cooperating associations: what's the difference?
Volunteers are individuals or groups of people who give their time, free of charge, to help the park either one time or on a regular schedule. Cooperating associations have a business relationship with the park or provide a specific service to the park. Some of them are not-for-profit groups which lease excess government buildings where they may run their own offices.
Friends organizations (see below) donate money and time to a national park or group of parks. Parks are forbidden by federal law to raise money or donations on their own, so friends organizations do this for us.