By default, WordPress allows some limited post filtering options using taxonomies. If you have many taxonomies and you need to filter posts taking into account several taxonomies, and to filter by more than one term, well, there is no direct way to that with WP only.
With WordPress 2.8 we got another interesting feature called custom taxonomies. You can make taxonomies from anything you want and use them much like you already use tags and categories. Unfortunately, themes offer only partial support (unsolvable problem) for custom taxonomies, but it’s not that complicated to implement.
