Terraria (Wiki): Wood Shelf

The Wood shelf is a solid surface designed for walking and traversing.

The Wood shelf is versatile, serving multiple purposes like bridging gaps without causing destruction, granting rooftop access while blocking certain enemies, or creating an elevated combat zone for players to shoot down at enemies while remaining untouchable.

The players can stand, walk, or jump horizontally on the Wood shelf, which functions similarly to regular blocks. Players can press the Down key while standing on the shelf to descend. However, these shelves are stable when landed on from above.

The Wood shelf consists of four unique placements. Players can alter their position using a hammer, similar to modifying a standard block:

• A first hammer strike will transform the structure into slanted stairs facing right. Additional visual supports beneath the slanted part may occasionally be present, depending on nearby blocks, but these are purely for visual purposes.
• A second hammer blow will cause the structure to switch to stairs facing left. 
• A third hammer blow moves the object to its "bottom" position, making it horizontal and lowered to the bottom half of the block. This allows for the attachment of banners and other hanging items beneath it. However, chandeliers or lanterns cannot be attached to this position (3DS and old-gen consoles).
• A fourth hammer strike resets the shelf to its original position. Note that only one shelf can be on a block tile at a time, so the top and bottom shelves cannot be on the same tile simultaneously.

When the object is in its default or "top" position, it occupies the upper half of the block space and allows furniture placement, such as Bottle, which becomes a Placed Bottle for potion making.

The Wood shelf is not lava-resistant compared to other shelves in the game; they will disintegrate if dropped, intentionally thrown into it, or upon contact. The Wood shelf supports gravity-affected blocks, such as Sand. Players can eliminate Wood shelves in a row with no negative effect on movement, just like regular blocks. Since the player occupies two tiles horizontally, they will not fall through gaps.

Staircases are versatile in orientation, regardless of the attached structure. The players ascend when facing the same direction as the stairs they climb, while stairs facing away from them have no impact. Players can bypass stairs by pressing the Down key, and the Smart Cursor feature allows for convenient positioning on shelves without needing a hammer. Stairs are vital for various advanced movements strategies like hoiks, impassable doors for NPCs, elevators, and enemy-catching traps.

The Wood shelf is capable of having hooks attached to it. While holding a shelf (3DS, old-gen consoles), players cannot grab any other object, regardless of the grappling hook's direction. The grappling hook will ignore any Wood shelves below the player's body (desktop, mobile, console). 

Ropes can now be integrated with the Wood shelf, and players will fall through shelves while riding a Minecart. Piggy Banks, Chests, crafting stations, and other types of furniture can be placed on Wood shelves just like regular blocks. However, there are a few exceptions, such as Gems and Wood Shards. Wood shelves can be used as shelves to store furniture in a limited space without limiting the player's movement.

Walking enemies can walk or pass through Wood shelves, while flying enemies like bats can fly upwards but not downwards through these shelves.

On the other hand, certain enemies like Harpies completely disregard shelves. Non-playable characters (NPCs), however, will always climb up shelves. Wraiths cannot ascend using shelves, but they can remain suspended above them.

Projectiles and weapons can pass through the Wood shelves.