All applications for trademark registration in the United States must undergo a very extensive examination process, which is conducted by Trademark Examiners at the U.S. Patent and Trademark Office. The Examiner conducts a limited search of existing trademarks and pending registrations to see if the mark conflicts with any prior rights. If so, the application is rejected. The Examiner also reviews the mark for its adequacy for registration as a trademark. This includes whether or not the mark is distinctive, and whether or not it is forbidden. If the mark is not distinctive, or if it is a forbidden mark, the Examiner will reject the application. (Note: marks rejected for lack of distinctiveness can be registered on the Supplemental Register after a separate filing). Additionally, the Examiner may reject the application for any number of formalities that have not been met, such as inadequate specimens, or a description of goods or services that is too vague. There is an extensive manual that the Trademark Examiners follow when processing applications, called the Trademark Manual of Examining Procedure that can be helpful if you have specific questions about how to treat a certain registration issue. Bitlaw has reproduced the manual with a keyword index and hyperlinks: http://www.bitlaw.com/source/tmep/wordindex.html.
