A tag is a word used to describe a bookmark in a social bookmarking web site, such as del.icio.us. Tagging refers to using a word, or string of words, to add information that can help organize online media so that it's easier to find.

Tagging is useful as it can help users find your content that is spread throughout the Web.  For example if you upload a video to YouTube, you'll want to tag it so that when a user performs a keyword query in YouTube, your video shows up in the results.