Wood Platforms are thin blocks through which players can jump or drop (using the 'S' by default). They are often used to build bridges and ladders, or to cover pits since they allow passage in both directions.
Wood platforms can be attached to a background wall, a block, or another adjacent Wood Platform (including one directly above or below). They cannot be attached directly to the background of the dirt or stone underground layers, since the backdrops of these areas don't count as "walls". However the sections 'anchoring' the platform to a wall can be destroyed after it is built out, leaving floating platforms.
When struck with a hammer, Wood Platforms can be turned into a set of stairs. Additional strikes can be done to lower them by half a tile. Any pickaxe can be used to remove them.
As of Terraria 1.2, it is possible to make an elevator by placing a column of two platforms and inverting one side of the column, as shown here: Youtube - Elevator Technology (unfortunately this is less effective as of 1.3)
Everything will pass through a Wooden Platform when coming from below, but some act different when touching from above. In the following situations, the top acts as a solid block:
Any wall-crawling enemy attempting to crawl through it.
On collision with most projectiles shot by monsters.
Placeable items can be placed on top of Wood Platforms. However, even if the platform was struck by a hammer to be placed on the lower side of the block, placing those items will still show them in the original location, as if the platform wasn't moved.
Lava will break platforms on contact, unless they're protected with water. In this case, lava and water won't mix into obsidian and platforms will separate the layers (see picture below). This can be used to make traps for underwater creatures, like the shark trap.
Wood Platforms can be grappled onto, but they let projectiles like arrows and bombs through.
Players can use Wood Platforms to build an arena to effectively fight many bosses, as projectiles pass through them, striking the boss while offering plenty of vertical and horizontal space for the player to maneuver in.
If a meteor strikes a skybridge made out of wooden platforms, the center platforms of the collision will be destroyed, and the outter platforms of the collision will be turned to meterorite.
Placing a platform two blocks down from any ceiling and then lowering/inverting it with a Hammer allows the player to jump on it and then subsequently through the ceiling.
Placing alternating columns of normal and half-tile/lowered/inverted platforms causes some odd behaviour; players, NPCs and walking mobs all seem to rise swiftly to the top of the column when passing from a normal column to an inverted column.