A trademark serves to identify a good or service, and to distinguish that good or service from others. The trademark may be a word, several words (such as a slogan), a design, symbol, graphic or any combination of these elements. Google and Coca-Cola (and their recognizable logos) are examples of trademarks.
While it is not required to register a trademark, it does provide the most protection for a brand. After researching whether or not a trademark already exists, they can be registered through the United States Patent and Trademark office (USPTO), which can be completed online.
Trademarks that are registered may use the symbol ®, while unregistered trademarks can be identified with a ™.
Sources:
“The Graphic Artists Guild Handbook: Pricing and Ethical Guidelines.” Eleventh Edition. Graphic Artists Guild, Inc. 2003.