I use them on my site. They are like a avatar that shows your rank or if you are a Admin, Modertaror or so on. I hope this helps you at all.
http://img297.imageshack.us/img297/854/aeonflux2copy5ts.png Thats a tag. Its peoples way of trying to make themselves sound more artistic and boosting their internet ego calling their signatures tags. The images to demote whether your admin/mod/etc. like said above are ranks, not tags. Tagnatures are simply the word combination of signatures and tags to make someone else's internet ego rise. In short, just call them signatures, they have been called that for years, no reason to change it lol