"A two-by-four. It comes in handy when something needs to be quickly crafted." -Item Encyclopedia
A Wooden Plank is simply a two-by-four made of wood.
A Wooden Plank is rather easy to craft with and is useful when wood is needed for a construction, but better items are usually availble to use for crafting. It can also be presumed that the Wooden Plank could be used as a substitute weapon, though it would likely be greatly inferior to any dedicated weapon in this case. A Wooden Plank looks like no more than a simple wooden board.
Alchemization and Crafting uses unknown.