If you have ever looked into getting a trademark for your product or service, you may have encountered the term "service mark" in addition to the term "trademark." What's the difference? The answer is simple in theory: trademarks are used to brand or distinguish a product, while a service mark is used to brand or distinguish a service.
But, as with all things legal, it can get a little more complicated than that. The term "trademark" can apply in two contexts: first, it refers to marks that identify a particular product. Second, it can be used colloquially to refer to all trademarks and service marks in general. In other words, all service marks are trademarks, but not all trademarks are service marks.
The purpose of trademarks (in the colloquial, general sense) is to identify the source of a product or service, and allow its owner exclusive use of the mark. This incentivizes the manufacturer or service provider to provide consistent, high-quality goods and services, lest their exclusive mark becomes associated with poor quality. The general public therefore benefits from higher quality goods in the marketplace. As a matter of policy, however, we don't want to give trademark owners too much protection, because that would allow companies to monopolize large swaths of the market, reducing competition and thus hurting consumers.
For these reasons, when you register for a trademark, you must identify the precise categories (classes) of goods and/or services your mark will be used on. As the mark's owner, you will only be entitled to legal protection for the use of your mark on those specific goods and services.
Here's where it matters whether you have a "trademark" or "service mark." If you only registered your mark for a service, you cannot also use it to identify a particular product, at least not with the registered symbol, ®. In order to use your mark on a broader spectrum of goods and/or services, you want to make sure you register your mark for the appropriate classes when you file.
Due to the complexities and nuances involved in selecting the proper parameters for your trademark or service mark, we always recommend consulting a licensed trademark attorney before registering your mark with the USPTO.