More great stuff from Mike.
There are some who have big problems with tags, because they're not specific enough, and they might overlap, and they tend to be a bit messy. What they do for us is allow us to categorize things for the task at hand. On Flickr, they help locate pictures of kittens, elsewhere they help categorize knowledge, websites, RSS feeds, etc. Tags are gritty and human, and just functional enough to get the job done. They serve as useful abstractions of the subject at hand.