The Posts panel includes a ‘Tags’ link. If you click on the link, you’ll see a page like this:
You may decide to use tags in a formal way, like categories and if so, this page is a useful way to add them, as well as a way to see how many posts use the same tag. Unlike categories though, tags can and should be added when you’re writing a post as they are meant to indicate keywords from what you’ve just written. They can be used to navigate your site (e.g. by using a tag cloud widget) and are valuable in that they help search engines understand what your content is really about. For that reason alone, you should always tag/keyword your posts.