Simply put, a service mark is a trademark used exclusively for services as opposed to goods. It is any word, symbol, phrase, picture, color, sound, packaging scheme or other "thing" used in commerce to identify and distinguish one service provider's services from another service provider's services.
Notice of a valid service mark is denoted by the use of the SM symbol, whereas the TM symbol is reserved for marks selling goods. It appears to be common usage to refer to service marks as either trademarks or service marks. However, the reverse seems to not be true. Nonetheless, the technically proper usage is to apply the SM symbol for services and the TM symbol for goods. While rare and unlikely, one can imagine where this distinction may have legal significance, such as when trying to establish constructive notice for a servicemark when both goods and services are offered in the same communication.