A trademark is a word, phrase, symbol or design that you use to identify and distinguish your goods from those of others. A service mark is similar to a trademark, but refers to services rather than goods. In practice, however, the term “trademark” is often used to refer to both trademarks and service marks.
Must all marks be registered? No, your use of your mark in commerce affords you common law rights to the mark. However, federal registration has several advantages, including notice to the public of your claim of ownership of the mark, a legal presumption of ownership nationwide, and the exclusive right to use the mark on or in connection with the goods or services described in your registration. Registration allows you to stop others from using a mark or trade name that is confusingly similar to your protected mark.
To register your mark, you must first file an application with the US Patent and Trademark Office. The best marks are fanciful, arbitrary or suggestive. The USPTO generally does not register generic terms. Examples of well-known marks include the Nike swoosh, Xerox and Windows, to name a few.
More information can be found at http://www.uspto.gov/trademarks/basics/.