“A word, name, symbol or device which is used in trade with services to indicate the source of the services and to distinguish them from the services of others.  

A trademark is the same as a service mark except that it identifies and distinguishes the source of a product rather than a service. The terms "trademark" and "mark" are commonly used to refer to both trademarks and service marks.

Trademark rights may be used to prevent others from using a confusingly similar mark, but not to prevent others from making the same goods or from selling the same goods or services under a clearly different mark. Trademarks which are used in interstate or foreign commerce may be registered with the Patent and Trademark Office.

The registration procedure for trademarks and general information concerning trademarks is described in a separate pamphlet entitled "Basic Facts about Trademarks.”

Source: United States Patent & Trademark Office

See also:
Brand Names, Product Names & Trademarks - What You Need to Know
How to Register a Trademark, Wall Street Journal