There are a handful of tagging plugins for wordpress that let you associate keywords (aka "tags") to your posts for indexing.
For the uninitiated, when you write a post using one of these tagging plugins, a new field appears in the editor that lets you specify tags that classify your post. These tags are gathered by other sites, like Technorati, and let you see all posts people have written that use the same tag.
So why is tagging different from using categories?
Some people would say it isn't any different, and I would say that they're outright wrong. But I say that because of how I segregate tags and categories mentally.