A trademark is a distinctive name, phrase, symbol, design, picture, or style used by a business to identify itself to consumers. If the business identified is a service rather than a product, the mark is called a service mark. In text and advertising one often sees the symbol ™ next to a phrase or image that a company thereby claims as a trademark, or the symbol ®, which signifies that the trademark has been registered with the appropriate government authority. Trademarking is a central legal component for corporate branding.
The intent of trademark law is to protect businesses from unfair competition and deceptive advertising by their competitors. For example, a company other than the Atlanta-based Coca-Cola Corporation that sold a soft drink called "Coke" in red and white cans would clearly be guilty of trademark infringement, confusing consumers as to the source of the product (though such confusion need not be intentional to be infringing). On the other hand, a company selling the carbonized coal byproduct called "coke" to steel mills would not likely be guilty of infringement, because there is little chance any consumer will mistake the two. The Atlanta company "owns" (in the intellectual property sense) not the word or design itself, but the exclusive right to use it to identify a particular product or service.
Unlike other forms of intellectual property such as copyrights and patents, trademarks must be actively used and defended. A copyright or patent holder may "sit on" his creation and prevent its use, but a company claiming (even registering) a trademark that fails to make active use of it, or fails to defend it against infringement, may lose the exclusive right to it. Further, if a court rules that a formerly trademarked term has become "generic" though common use (and so the average consumer doesn't realize it is a trademark), it may also be ruled invalid. For example, the Bayer company's trademark "Aspirin" has been ruled generic, so other companies may use that name for their products as well. Trademarks do not expire; they are valid as long as they are actively in use.