Roxas, that's a stick.
Xion to Roxas during Mission 31.
The Wooden Stick is a weapon Roxas can wield in Kingdom Hearts 358/2 Days. When Roxas and Xion go on a mission to Beast's Castle, Roxas lends Xion his Keyblade when she can't use her own, and Roxas takes the stick for himself. The stick's weakness is displayed in the battle with the Bully Dog at the end of this mission, in which it breaks slightly while Roxas attacks. The stick is near-useless as a weapon, but Roxas is still able to use magic while wielding it. Since he is fighting alongside Xion, Heartless will still release hearts.
The Wooden Stick is nearly identical to the stick Roxas later uses at the Simulated Twilight Town in Kingdom Hearts II during the second day of his story. He picks it up, mimicking that he is in possession of the Keyblade before throwing it over his shoulder, causing it to pathetically bounce off Riku, who was standing behind Roxas. On the final day of Roxas's story, the stick is seen again, and Roxas picks it up off the ground before it turns itself into the Kingdom Key.
The Wooden Stick is, to put simply, a stick. It is a light-brown color, and has a slightly curved shaft. One side of the stick has a small twig jutting out of it, mirroring the teeth of a normal Keyblade. The Wooden Stick is only a small amount shorter than the Kingdom Key.
The design of the stick in Kingdom Hearts II is near identical, only much more crooked, the twig is shorter, and the stick in general is considerably smaller.
The stick's basic combo is identical to the Kingdom Key's aerial and ground combo.
Interestingly, when Sora uses the Wooden Sword on the Destiny Islands in Kingdom Hearts, it just goes right through the Heartless without doing damage. Later, though, in Hollow Bastion, it can do damage, albeit an amount as to be almost unnoticable, forcing the player to either use magic or rely on Beast to do damage. The Wooden Stick may be a reference to having to rely on your partner as you did in Kingdom Hearts (and, coincidentally, both worlds where you're forced to use a wooden weapon feature Beast)