I was talking with one of my clients and this topic came up.  In conversations I have has since then, it is apparent that the most people do not know that there is a difference between an agent and a Realtor (I would like to point out that Realtor is a registered trademark of the National Association of Realtors, NAR). 

A real estate agent is a liscensed salesperson in the state(s) they work in.  They must abide by the laws of the state, nation and most likely the locale of the area.  An agent may or may not be a Realtor.  Some local multiple listing services are owned or operated by the local board of Realtors, and as such all member agents must be a Realtor. 

A Realtor is a member of the NAR and also a member of their local branch of the association.  Realtors have a code of ethics they subscribe to.  The code includes principles such as loyalty to the client, legal duty to the client, cooperation with other agents, and truthfulness in all statements. 

This is not to say that agents that are not Realtors are not good agents or people, but by becoming a Realtor, the agent has put an additional mark that they promise to meet.  When choosing an agent, this is something you should research and consider before working with any particular agent.  Ask them if they are a Realtor, and if not, why?