The 5 Planes of Terraria

Plane # 1 --

This plane contains the sun, moon, sky, clouds, and stars. It is the background of the game and cannot be interacted with. The movement of objects in the plane are affected by your parallax setting.

Plane # 2 --

This is the plane with the background walls in it that your character can pass over. you can place and destroy things on the plane but other than monster spawning and aesthetics they do not effect anything.

Plane # 3 --

This is the plane where all objects that can be passed through, are in front of walls, but can be interacted with lie.

IE- Work Bench, Chest, Tree, Vine, Furnace, Torch, Chain Lantern, Book, Pot, Open Door, Candle, Chair, Any plant except for Man Eaters and the spiky ones that hurt you. (Tables have a surface you can jump on similar to platforms and platforms are considered blocks.)

Also, this plane contains any object that has been dropped, because if your inventory is full, it will act exactly the same as everything else in this plane.

Plane # 4 --

This plane contains most of Terraria within it. It contains every single block and liquid within it. Every monster in the game that doesn't pass through walls, such as Zombies, are in this plane. All the NPCs rest here. Also, the player is in this plane, but he/she can interact with every plane except for the first.

Plane # 5 --

The fifth plane contains things which can go in front of things that are on the fourth plane like blocks. Some of what it contains are: