What are Tags?
A tag is a non-hierarchical keyword or term assigned to a piece of information (such as an internet bookmark, digital image, or computer file). This kind of metadata helps describe an item and allows it to be found again by browsing or searching. Tags are chosen informally and personally by the item's creator or by its viewer, depending on the system. (Source: Wikipedia)
All your assigned Tags are listed in the main menu "Tags" or in "MY Bookmarks" on the right side. If you select on eof these Tags your Bookmarks will be filtered by it.
On this place I would like to thank Delicous and Flickr for making tags popular on the web.