A service mark is used to identify particular services. For example, a car wash could register a servicemark for their car wash services: “BlueWater” Car Wash.
A service mark is technically defined as “any word, name, symbol, devices, or combinations thereof used by another to distinguish services of one person from those of another, and to indicate the source of that service even if the source is unknown.”
Those service marks not federally registered commonly use the “sm” symbol as opposed to the circle “R” (®) which may only be used if the service mark is federally registered.