Adds the Description and Keywords XHTML META tags to your blog’s front page and to each one of the posts, static pages and category archives. This operation is automatic, but the generated META tags can be fully customized. Also, the inclusion of other META tags, which do not need any computation, is possible. Please read the tips and all other info provided at the configuration panel.
Plugin Documentation: http://wordpress.org/extend/plugins/add-meta-tags/