To use the word "Realtor" you have to be a member of the National Association of Realtors, which is North America's largest trade association, with well over a million members as of 2006.
The NAR has members in every part of the real estate business. From brokers and agents to property managers and appraisers. Every member of the NAR has pledged to a code of ethics and Standards of Practice, which includes duties to clients, the public, and other Realtors.