From Wikipedia:

"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."

The "TM" mark is reserved for unregistered trademarks. A registered trademark is an "R" inside a circle.