The owner of a trademark is responsible for protecting it from use by others, which requires actively monitoring the use of the mark, and confusingly similar marks, being used in commerce. Effective monitoring can be accomplished by the regular periodic review of a proper trademark search by an attorney.
If infringement is identified, US law makes it convenient for owners of registered trademarks to sue for damages and/or injunctive relief (a court order that the infringer stop using the mark). Because such lawsuits are quite effective when brought by the owners of registered trademarks, a cease and desist letter may be a sufficient enforcement tool.
If you learn that someone else is using your registered trademark, or has registered a similar trademark, you may lose your own trademark rights if you do not assert them against the other party.
If you receive a cease and desist letter, it is advisable to speak with an attorney to evaluate the merits of the letter and discuss options to resolve the dispute so that both parties may continue using the mark.