According to Wikipedia:
A tag is a (relevant) keyword or term associated with or assigned to a piece of information (a picture, a geographic map, a blog entry, a video clip etc.), thus describing the item and enabling keyword-based classification and search of information.

Tags are usually chosen informally and personally by item author/creator or by its consumer/viewers/community. Tags are typically used for resources such as computer files, web pages, digital images, and internet bookmarks (both in social bookmarking services, and in the current generation of web browsers - see Flock). For this reason, "tagging" has become associated with the Web 2.0 buzz. Many people associate "tagging" with the idea of the semantic web, however some believe that tagging may not be having a positive effect on the overall drive towards the semantic web.

In the XDe various modules have been integrated with the global tagging module that allows tags to be added to items in those modules thereby facilitating searches and browsing for users both within the module and overall for the web site.

Tags are displayed along with the item or as a "Tag Cloud" A tag cloud is a set of related tags with corresponding weights (based on how many times the tag appears). Typical tag clouds have between 30 and 150 tags. The weights are represented using font sizes or other visual clues (bold). Also, tag clouds are interactive: tags are hyperlinks that allow the user to drill down on the data.

When assigning a tag to an item in any module try to use a consistent name for tags relating to the same thing - this will build the ´weight´ of that tag and make it work better as an item in the cloud.