Tagging, also known as labeling, is the ability to add your own keywords to a piece of content, which will allow others to then reference the content based on similar keywords. The best known example of this is del.icio.us, but the technology is also used on sites such as House of Fusion (though it is generally ignored.) This technical article by Kunal Anand shows one approach to tagging along with example code. He hopes that this should be enough for people to start building their own tagging implementations.
Tagging Engine Implementation in ColdFusion (Kunal Anand's Blog, December 2005)