Look closely at product packages and advertisements, and you're likely to see the "TM" symbol all over the place. The symbol serves as a company's assertion that a name, logo or something else is a trademark. The "TM" doesn't provide as much protection as a federally registered trademark, but it is still powerful.
A trademark can be just about anything that identifies the products that one company makes and distinguishes them from products made by competitors. Brand names, logos, symbols, slogans, color schemes, designs and other elements can all be trademarked. Trademarks only apply to goods -- physical products. Businesses that provide services get the same protections from service marks.
When a company uses the "TM" symbol with a name, logo or other signifier, it is claiming the right to use the mark. Anyone can use the "TM," since it is not regulated by the U.S. Patent and Trademark Office. Use of the symbol does not guarantee that you will win any fights over the right to the trademark -- especially if someone else is claiming the same mark or a similar mark and has been using it longer than you have -- but it immediately and publicly establishes your claim. If no one else is using a mark, the "TM" plants your flag there first, which carries a lot of legal weight.
In the United States, the only way to establish a trademark is by using it. If you owned, say, an auto parts fabricator and wanted to trademark "Arizona's Best Lug Nuts," the first step would be to make sure no other business was already advertising itself with that phrase. If no one was, then you could start using the phrase in commerce -- in your advertising, promotions, signage and so on. And you would use the "TM" symbol to claim rights to the mark.
To gain a full range of legal protections for a trademark, it's best to register it with the U.S. Patent and Trademark Office. This gives you the right to use the registered trademark symbol -- the familiar "R" in a circle. U.S. intellectual property law recognizes trademarks even if they're unregistered -- that is, "TM" trademarks -- but registration provides several advantages. For one, it serves as public notice of your trademark, so that someone conducting a trademark search will see it. It also creates the legal presumption that you have the rights to the mark, which can help you in legal disputes over trademarks, and it gives you the right to sue in federal court for trademark infringement. The U.S. Patent and Trademark Office requires that a company be using a trademark in order to register it; a company can't register a stockpile of trademarks "just in case" it wants to use them in the future.