A trademark is a distinctive word, phrase, logo, domain name, graphic symbol, slogan, or other device that is used to identify the source of a product and to distinguish a manufacturer's or merchant's products from others. Some examples are Nike for sports apparel, Gatorade for beverages, and Microsoft for software.
A service mark does the same thing as a trademark, but while trademarks promote products, service marks promote services and events. Some familiar service marks are: Google (online searching services), Netflix (video rental service), and the FedEx logo (delivery services).
In order to be eligible for trademark protection, a word or phrase must be "distinctive" -- unique enough to help customers recognize a particular product in the marketplace -- rather than generic, like "The Coffee House." To determine whether a potential business name or product name is trademarkable, and how to trademark it, see Trademark: Legal Care for Your Business & Product Name, by Attorneys Stephen Elias and Richard Stim (Nolo), or the Qualifying for Trademark Protection FAQ.