I've wondered what the difference between a tag and a category is before too. Posts can have multiple tags, and they can be filed under multiple categories, so what's the point? As near as I can tell, the only difference is that tags are more specific and intended to be used to search for similar posts, while categories are broader and meant to be used for browsing. But it's entirely up to the creator of the content to tag/categorize the posts in that way. The only use I've found for tags are on large sites (namely Engadget) that make a lot of posts a day, but I only care about things from a few tags, so I just subscribe to those tags.