A trademark is a word, symbol, shape, or any sign affixed to a product that indicates the source of the product and distinguishes it from others. A servicemark is the same as a trademark except that it identifies and distinguishes the source of a service instead of a product. The terms "trademark" and "mark" are commonly used to refer to both trademarks and servicemarks.

Generally, a trademark consists of a name, word, phrase, logo, symbol, design, image, or a combination of these elements. Trademark status may also be granted to distinctive packaging, color combinations, building designs, product styles, and overall presentations. The Bass Red Triangle was the first trademark registered in Britain in 1876.

Trademarks allow businesses to prevent others from using a similar mark, so that consumers know the origin of the goods they are purchasing. Other businesses can still make or sell the same or similar goods or services as long as they use a different mark to identify them. Trademarks, therefore, protect consumers, who know what they are buying, as well as provide an incentive for manufacturers or suppliers to consistently supply high-quality goods or services.

Both federal and state laws cover trademarks in the United States. Congress has authority to regulate interstate and foreign commerce under the U.S. Constitution, Article 1, Section 8, Clause 3, sometimes called the Intellectual Property Clause. It used this power to enact the Lanham Act, 15 U.S.C. §§ 1051 - 1127, which governs trademark use at the federal level. A trademark registered under the Lanham Act has nationwide protection under Section 1115 of the Lanham Act.  Trademarks are also protected by the Tariff Act of 1930, 19 U.S.C. Section 1526.

Unfair competition laws at the state level protect trademark rights and registration of marks is not required. State laws governing trademarks vary but most have adopted a version of either the Model Trademark Bill (MTB) or the Uniform Deceptive Trade Practices Act (UDTPA). The MTB provides for registration of trademarks, while the UDTPA does not.

A trademark is considered to be a piece of property, so the law provides certain rights protecting it. These rights can be established either by use of the mark in the marketplace or by registering it with the U.S. Patent and Trademark Office (USPTO). A registered trademark confers a bundle of exclusive rights upon the registered owner, including the right to exclusive use of the mark in relation to the products or services for which it is registered.

If a trademark has been registered with the USPTO, the owner may file suit in federal court to enforce his exclusive rights through an infringement action. Infringement may occur if another entity is using a registered trademark unknowingly for its own products or services, or intentionally to capitalize on the goodwill of the registered mark. A court will consider the similarity of the trademarks involved, the similarity of the products and/or services involved, and whether the owner's trademark is well known.

Use of a trademark is indicated by affixing the ™ symbol to a product, even if that trademark has not yet been registered with the may be used when a business seeks to claim trademark rights, but the mark has not been registered with the Patent and Trademark Office. The ® symbol can be used to indicate that the mark has been registered. It is not mandatory to use either symbol, but both are prevalent around the world and give notice to the public that a mark is in use.

A holder who has registered his mark may use the U.S. registration as a basis to obtain registration in foreign countries, and the ability to file the U.S. registration with the U.S. Customs Service to prevent importation of infringing foreign goods. There is no way to obtain a trademark registration that will automatically apply worldwide. Trademark laws vary in different countries, although an increasing number of systems and treaties are being developed to protect trademark rights in a more uniform manner internationally.

Trademarks, unlike copyrights and patents, do not expire but remain valid as long as the owner actively uses and defends them and maintains their registrations. A trademark can be abandoned, or its registration cancelled, if the mark is not continuously used. Further, if a mark owner does not assert his right to use the mark exclusively, it may become a generic term and no longer a protected mark.

Generic terms may be used by anyone, and many former trademarks have become generic, including escalator, aspirin, yo-yo, zipper, and trampoline. Through lack of enforcement by mark owners, these terms lost enforceability as marks and became generic. Today, Xerox as a term for copiers and Band-Aid as a term for adhesive bandages risk becoming generic; those companies must (and do) actively seek to prevent others from using those marks to refer to their own copiers or bandages in order to maintain their trademark rights.