Realtor is a registered trademark of the National Association of Realtors, referring to people who sell real estate - buildings or land - and who are members of the association.
It is legal in the U.S. to sell real estate without being a Realtor.
As a trademark, the term should always be capitalized, as compared to once-trademarked terms such as "aspirin" which have entered the public domain and are no longer protected by trademark status.
