Credit Unions are privately owned and managed organizations that are owned by the members that contribute funds to it. This means that each member in a credit union is also owner of it. They are run by the objective of promoting thrift, providing financial assistance like loans and credit cards to its members and are usually not for profit organizations. The policies of a credit union and the day to day working are monitored by a Board of Directors and usually the board is made up of volunteers.
The differences between a bank and a credit union are not very apparent because both these organizations offer similar services, however banks are usually much larger and will have a much broader range of services than a credit union among other differences.
Credit unions are also popular because once you are a member of a credit union it becomes easier to get a credit card or a loan and this is usually a very good way to build credit history.