A real estate agent works for a broker or real estate brokerage firm and has a license to help sell and buy real estate. Brokers are also real estate agents, but they have had extra training and have additional licensing so they can work independently and/or hire real estate agents to work under them.
Real estate agents and real estate brokers can both be Realtors. A Realtor is licensed by the state as a real estate agent/broker and is also a member of the National Association of Realtors (NAR).