U.S. Patent and Trademark Office (USPTO): 

A trademark includes any word, name, symbol, or device, or any combination, used, or intended to be used, in commerce to identify and distinguish the goods of one manufacturer or seller from goods manufactured or sold by others, and to indicate the source of the goods. In short, a trademark is a brand name. 

Patents protect inventions and improvements to existing inventions.
Copyrights cover literary, artistic, and musical works.
Trademarks are brand names and/or designs which are applied to products or used in connection with services. 

PURPOSE:

Trademark law is designed to fulfill the public policy objective of consumer protection, by preventing the public from being misled as to the origin or quality of a product or service. By identifying the commercial source of products and services, trademarks facilitate identification of products and services which meet the expectations of consumers as to quality and other characteristics. Trademarks may also serve as an incentive for manufacturers, providers or suppliers to consistently provide quality products or services in order to maintain their business reputation. 

IDENTIFICATION:

A trademark or trade mark, represented by the symbols ™ and ®, or mark, is a distinctive sign or indicator used by an individual, business organization or other legal entity to identify uniquely the source of its products and/or services to consumers, and to distinguish its products or services from those of other entities. A trademark is a type of intellectual property, and typically a name, word, phrase, logo, symbol, design, image, or a combination of these elements. 

LEGAL:

The owner of a registered trademark may commence legal proceedings for trademark infringement to prevent unauthorized use of that trademark. However, registration is not required. The owner of a common law trademark may also file suit, but an unregistered mark may be protectable only within the geographical area within which it has been used or in geographical areas into which it may be reasonably expected to expand. 

USC TITLE 15 > CHAPTER 22 > SUBCHAPTER II > § 1095 says: "Registration of a mark on the 'supplemental register' shall not constitute an admission that the mark has not acquired distinctiveness." 

SYMBOLS:

The two symbols associated with U.S. trademarks ™ (the trademark symbol) and ® (the registered trademark symbol) represent the status of a mark and accordingly its level of protection. While ™ can be used with any common law usage of a mark, ® may only be used by the owner of a mark following registration with the U.S. Patent and Trademark Office (USPTO or PTO) and designates such. The proper manner to display either symbol is immediately following the mark, preferably in superscript style. 

TERMS:

Terms such as "mark", "brand" and "logo" are sometimes used interchangeably with "trademark". "Trademark", however, also includes any device, brand, label, name, signature, word, letter, numerical, shape of goods, packaging, combination of colors, or any combination thereof which is capable of distinguishing goods and services of one person from those of others. It must be capable of graphical representation and must be applied to goods or services for which it is registered. 

THE LAW:

The law considers a trademark to be a form of property. Proprietary rights in relation to a trademark may be established through actual use in the marketplace, or through registration of the mark with the trademarks office (or "trademarks registry") of a particular jurisdiction, e.g., the U.S. Patent and Trademark Office. 

The Lanham Act, (USCODE TITLE 15, CHAPTER 22, SUBCHAPTER II, § 1094) says: "...no other person has the right to use such mark in commerce either in the identical form thereof or in such near resemblance thereto as to be likely, when used on or in connection with the goods of such other person, to cause confusion, or to cause mistake, or to deceive... You may want to read this section of the law as well. 

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. The law in most jurisdictions also allows the owner of a registered trademark to prevent unauthorized use of the mark in relation to products or services which are identical or "colorfully" similar to the "registered" products or services, and in certain cases, prevent use in relation to entirely dissimilar products or services. The test is always whether a consumer of the goods or services will be confused as to the identity of the source or origin. Read more in popup window about the Lanham Act on law pertaining to trademarks. 

USAGE:

The extent to which a trademark owner may prevent unauthorized use of trademarks which are the same as or similar to its trademark depends on various factors such as whether its trademark is registered, the similarity of the trademarks involved, the similarity of the products and/or services involved, and whether the owner's trademark is well known. 

If a trademark has not been registered, some jurisdictions (especially Common Law countries) offer protection for the business reputation or goodwill which attaches to unregistered trademarks through the tort of passing off. Passing off may provide a remedy in a scenario where a business has been trading under an unregistered trademark for many years, and a rival business starts using the same or a similar mark. 

RECOURSE:

The Ninth Circuit Court has recognized that Trademark Holders could satisfy their burden in one of three ways: 
(1) it has a federally registered mark in goods or services; or 
(2) its mark is descriptive but has acquired a secondary meaning in the market; or 
(3) it has a suggestive mark which is inherently distinctive and protectable. 

The 11th Circuit Court of Georgia ruled the use of others' trademarks in meta tags infringes trademark laws, according to North American Medical Corp. v. Axiom Worldwide, Inc. (April 7, 2008) (PDF) 

If a trademark has been registered, then it is much easier for the trademark owner to demonstrate its trademark rights and to enforce these rights through an infringement action. Unauthorized use of a registered trademark need not be intentional in order for infringement to occur, although damages in an infringement lawsuit will generally be greater if there was an intention to deceive. 

For trademarks which are considered to be well known, infringing use may occur where the use occurs in relation to products or services which are not the same as or similar to the products or services in relation to which the owner's mark is registered.