"A service mark is any word, name, symbol, device, or any combination, used, or intended to be used, in commerce, to identify and distinguish the services of one provider from services provided by others, and to indicate the source of the services. A service mark is processed and treated substantially the same a TRADEMARK." (1)
"A TRADEMARK is either a word, phrase, symbol or design, or combination of words, phrases, symbols or designs, which identifies and distinguishes the source of the goods or services of one party from those of others. A service mark is the same as a trademark except that it identifies and distinguishes the source of a service rather than a product." (2)
This means that, whereas a trademark identifies one party's goods or services as unique from another's, a service mark only identifies services. A service mark will normally appear in advertising for the service.
As with the rest of trademark law, the use of service marks is pretty hairy. You are not required to register your service with the PTO to use a service mark, but registering can grant you benefits not available otherwise — such as entitlement to use the mark nationwide. The first party to use a service mark is generally who has the right to register the mark, but things get sticky when two separate parties use the mark in similar manners, without either knowing about the other party, and without either holding a federal registration to the mark. In the long run, a court decision has the final say in matters regarding who has the right to use a service mark.
You can search the Georgia Secretary of State's database of trademarks and service marks at http://www.sos.state.ga.us/corporations/marksearch.htm.
The service mark is Unicode entity 2120, in the Letterlike Symbols block (along with "TM"): ℠
References:
1: smith & hopen, p.a.: http://www.baypatents.com/
2: The Offices of Alex D. Patel: http://www.tmlawoffices.com/
I am not a lawyer.