Classes give you special items upon spawning in, all of them tailored towards a certain style of play. They cost Shotbow xp to unlock, although if you buy a class for a day and never change it, you can have that class forever.
The default class; it has no special item, allowing you to carry one extra item.
Recieves a special Redstone Block which can destroy and regenerate terrain
Has a wrench that can repair cars and mechs
Selecting Spec Ops gives you the grapple; a fishing rod which has the ability to pull you onto buildings and fortifications, making it great for stealthy players. In order to use the grabble, you need to aim, right click, wait for the grapple to hook, then right click again.
Scout gives you a watch which will make you invisible for a short period of time. Right clicking the watch gives you Invisibility 2 for 7 seconds, which makes your body invisible, but not your selected weapon or hat.