There are two types of movable atoms: objs and mobs. The difference between them is that a mob can be attached to a human player, and is also typically used for NPCs and creatures. The obj type is a little bit simpler and is typically used for objects in the environment, items in inventory, etc.
Objects are derived from /obj, which derives from /atom/movable.
The following example defines the obj type /obj/scooper.
obj
scooper
desc = "Super pooper scooper."