A content tag, in content management system application, is a term or phrase that is added to a web publication that relates the specific publication to others that are similarly tagged within the system. 

The terms content tag (or tag) and keyword are often used interchangably, but there are differences between the two. 

A Keyword Meta Tag is meta data that is placed inside the HEAD component of an HTML document.  This meta element is typically invisible to an audience. It also was widely misused by many unscrupulous web administrators who would stuff meta tag keywords with large amounts of popular, yet irrelative, terms in an effort to drive web traffic to their sites.  Due to this, the Keyword Meta Tag is not used as a primary element for search engines anymore. 

A Content Tag, on the other hand, is typically very visible to an audience.  In fact,  this type of tag will often appear as a hyperlink that users can click on to view other content in the system that contains the same tag.  Content tags are not necessarily tools that are utilized for search engine optimization.  Instead, they are tools that are used within a content management system to organize, filter, and relate content for end users.

Tags can generally be applied in a few different ways, depending on the system that is using them.  Some systems will allow for the creation of highly controlled tagging lists that content providers can choose terms from.  Other systems may supply a free-tagging method, where users just type in terms.  Some systems allow for both methods.