Trademarks are words, phrases, symbols or designs that identify the source of goods of one party from those of other parties. A service mark is the same as a trademark, except that service marks identify and distinguish the source of a service rather than goods.
Rights to a trademark are established by using the mark in commerce. Registration of the mark with the U.S. Patent and Trademark Office is not mandatory. However, registration of a trademark provides a very important advantage to the owner. If you want to sue someone for using the mark without permission, it must be registered in order to collect statutory monetary damages under federal law.
Failure to register does not preclude so-called “common law” rights under state law, but you must prove actual damages from the infringing use instead of collecting statutory damages. As a practical matter, actual damages can be problematic to prove at trial.