What it is:
A tag is a piece of metadata (e.g., a keyword or term) assigned to a piece of digital information (such as an internet bookmark, image, or computer file). Tagging is classifying content for other uses later, including "re-finding" it and sharing it. Tagging is closely related to social bookmarking.
Tagging means to label with a keyword, usually in a blog or on a website. Tagging allows users to classify and find information and it is a characteristic of Web 2.0 services. For example, you might tag a photo in Flickr, or tag a bookmark in Delicious.