WordPress features categories and tags. They’re basically the same, but a post can only be in one category and have multiple tags. So, as categories are like the ugly stepbrother of tags, you should use tags to identify what your blogpost is all about. Many plugins like the Efficient Related Posts plugin rely on tags, and tags are also good for your Google-and-other-search-indexer ranking, as these are additional words people might have searched for.
What do we gain by using tags? The above plugin gives me the biggest reason to use tags. As I describe with them what content is in the text, I can make sure readers get presented more posts with similar topics. This might help the reader and might make them want to read more stuff. Which is really nice. But there’s more. I just used the Simple Tags – WordPress plugin to rename my formerly german-spelled tags (like “Philosphisches”) to their english equivalent (like “Philosophy”). This way, I can divide between my english and german posts more. I also tagged all my english posts as “english” and all my german posts with “german” and “deutsch”, this way the related posts plugin will most likely give you more related english posts if you read an english post and vice-versa. Additionally, everyone can now use a URL like http://www.ithoughts.de/tag/German to only see all posts tagged with the certain key word … using the above tag, my visitors can read all posts written in german.
Use Tags. Even if Category is a nicer and more understandable term, tagging is what you want to do – and what is supported more.