A trademark is a word, name, symbol, or device that is used in trade with goods to indicate the source of the goods and to distinguish them from the goods of others. A servicemark is the same as a trademark except that it identifies and distinguishes the source of a service rather than a product. The terms “trademark” and “mark” are commonly used to refer to both trademarks and servicemarks.
Trademark rights may be used to prevent others from using a confusingly similar mark, but not to prevent others from making the same goods or from selling the same goods or services under a clearly different mark. Trademarks that are used in interstate or foreign commerce may be registered with the USPTO. The registration procedure for trademarks and general information concerning trademarks can be found at basic facts about Trademarks.
Words, phrases, symbols, designs (or a combination of those things) that distinguish a product or service from other kinds of products and services. The mark has to be unique: it can't look like, sound like, or mean the same thing as an existing mark (if it does, that's called "likelihood of confusion").
Unlike patents, you don't have to apply for a trademark right away: sometimes it takes years to achieve the brand recognition you need to argue that your logo/color/name/etc is distinct enough to be trademarked.
Three symbols are commonly used to indicate trademark protection: