Trademark vs. Service Mark
Simply put, trademarks are for goods while service marks are for services. When discussing either, it is common to use the term "trademark", even when discussing a service use, because the handling of either is interchangeable by both the USPTO and all 50 Secretary of State Offices.
The USPTO says that a "service mark is the same as a trademark except that it identifies and distinguishes the source of a service rather than a product. The terms 'trademark' and 'mark' are often used to refer to both trademarks and service marks."
Let's look at each term in more detail
The USPTO's definition: trademark "protect words, names, symbols, sounds, or colors that distinguish goods and services from those manufactured or sold by others and to indicate the source of the goods."
If your name and/or logo appear on the tangible goods that you're selling, you'd be filing for a trademark. For instance, let's say you want to protect the name of your clothing line. As long as the name appears on the hang tag, label or the packaging the clothes come in, that would suffice as proof of you using the name in connection with a clothing line. And therefore, you'd file for a trademark.
If the name appears only on the front of the shirt, that's ornamental use and therefore not eligible for trademark protection.
The USPTO's definition: service mark is "a word, name, symbol or device that is to indicate the source of the services and to distinguish them from the services of others."
If you're selling services in connection with a name and/or logo, you'd be filing for a service mark. TradeMark Express is our service mark that we use in connection with our trademark research & application services.
For example, you are opening a restaurant using a specific name. The name as it appears on any signage, menus, advertising, etc., would suffice as proof of your use in connection with your services.