I've tried to rate these according to THE authority on level reviews, Hyperview. However, if another site has a rating that Hyperview doesn't, I'll post that. They are small and don't involve a lot of COGs like SP levels do. They involve new weapons, modified weapons, partial conversions, total conversions, and anything else that doesn't fit into the other categories. Most of the things people send in to File Database are mods.
Skins are the broad category under which new player skins and models fall under.
You'll rarely find a player model under this heading - they are usually packaged with skins, sounds, and more on the Mods page. We get less and less of these over the months. The files in the essentials section are mostly based on popular opinion and use.