First there are just plain old objects, you can animate, script, make them pushable and other cool things. Anything complicated not using bots(NPCs) will probably require these.
Then there are the bots, generally meaning anything you can kill or talk to.
Terrain is for walls, stairs, roofs, floors and other simple things.
Items are just stuff you can place in your inventory or on ground you'll need to edit items.ini file to change them in any way.
Lights prevent the map from being completely dark, of course objects can do light too and there's sunlight option in the map settings.