A trademark is generally a word, phrase, symbol, or design, or combination thereof, that identifies and distinguishes a source of goods of one party from those of others.
A service mark is similar a trademark, except it identifies and distinguishes the source of a service rather than goods.
The terms "trademark" and "mark" refer to both trademarks and service marks.
A trademark usually protects the brand names and logos used on goods and services.
Trademark is not the same as copyright or patent. Copyright protects an original artistic or literary work. A patent protects an invention.
For more information about trademarks, please see the USPTO's "What is a trademark?" and the Trademark Basics Registration Toolkit.