I pre-ordered an 18 slot ranked server today. Basically the ranked servers are more 'official' so you can't change as much. You can choose a game mode, map, kick players, etc. When players play in a ranked server, they can earn XP, unlock challenges, basically anything that you would expect to be normal.

In an unranked server, you can tweak whatever you want, think modded lobbies, however, those are just for fun, and don't count towards player stats or XP at all. It has all the features of a ranked server, plus you can alter advanced settings, like player health, toggle HUD and other settings, load mods and stuff.

If things go well and we decide a modded server would better suit our needs then we will look into changing over.