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 have been trying to resolve an odd problem with my nginx config, which seems to have stopped working shortly after applying a Debian backports upgrade. The landing page of my blog ...