A Credit Union is like a bank that the people who deposit their money own. They vote regularly to elect the board of trustees and sometimes administrators of the Credit Union. (Depending on if they are under Federal or State law in the US.) They can only accept deposits from some limited group of people and likewise they can only loan money to that same group and invest in certain safe things like US Government Bonds. They also must have their reserve deposited at a bank that is a member of the Federal Reserve System.
Most are based around employment in some large company, being a relative of a person who was or is an employee, or is associated with some group.
A few examples:
Anheuser-Busch Employees' Credit Union
Coors Credit Union
Colorado State Employees Credit Union
Florida State University Credit Union
Johns Hopkins Federal Credit Union
Portland Teachers Credit Union
Most are excellent institutions. I highly recommend joining one if you can. Check the lists on Yahoo or other sites to see if you are qualified for one. They generally pay interest on all checking accounts, with no minimum balance, or charges. Also they typically pay higher rates of interest to depositors and charge less for loans.