A permanent link to an item (a blog post, an attachment, ...). The default structure is "?p=123", but WordPress can handle "pretty permalinks" and will rewrite them internally to the former version. If you have a question about special rewrite structures you should use the [url-rewriting] tag.
I am working on a custom theme for a network of sites that are deployed under a WordPress MultiSite installation. Within these theme I create 4 custom post types that have two taxonomies added to all ...