A trademark consists of a symbol, design, word or group of words that establishes a relationship between a business and the products it furnishes. It may contain words only, a logo or a combination of words and a logo. A servicemark is similar to a trademark, but it associates a business with the services it provides. The term "trademark" is often used interchangeably for both trademarks and servicemarks.
Trademarks are a type of intellectual property. They are used to uniquely identify a business, using the logo image and combination of words as a type of signature. Under the protections, other parties are barred from using imagery that is similar to a trademark, opening the infringing party to possible litigation. However, a trademark does not prevent a competitor from marketing a similar product.
Trademark rights can be registered with certain government entities to formalized the protections. However, this step is not necessary; the right to exclusive use of the imagery or group of words can be asserted if the holder can demonstrate that its use in a commercial setting is legitimate. These claims may be asserted when using the image by including a "TM" designation.
In some cases it might be beneficial to have a trademark that is official registered with the government. Many state governments have a registry, and entities that are doing business across state borders and in other countries are able to register with the U.S. Patent and Trademark Office. After the mark has officially been included in the federal registry, the claim to it can be asserted using the official federal registration symbol.
Like other types of intellectual property, a trademark can be a valuable asset to a business. However, in some cases, outside parties might attempt to use a trademark without permission, which may negatively affect the owner of the property. If this occurs, the trademark holder might pursue litigation in an effort to seek damages.
Source: Findlaw, "What is a Trademark?", September 04, 2014