A certification mark is a type of trademark used to certify the nature of goods and services that are not provided by the mark's owner. Some examples:
Although a single entity is given ownership of the certification mark, they must state a standard for certification, and must permit anyone who meets that standard to use the mark. This means that the mark holder cannot arbitrarily discriminate between applicants. If someone can prove that the mark is arbitrarily granted or inadequately policed, they can use the certification mark on their own products and raise inconsistency as a defense to any subsequent infringement claim. (Warning: Don't do this without consulting a lawyer first.)
Certification marks are one of the oldest types of trademark, as they date back to the days of medieval guilds. Personal trademarks didn't appear until much later in legal history.
A related concept is the collective mark, which is held by a group for use by the group's members. | 
