Scribblenauts is basically a 2D puzzle game where you solve the puzzles using items you can summon by typing their name. For example you have to get a star that is on top of a tree. You can type in "ladder" and use it to climb the tree or "axe" to cut the tree down or "jetpack" to fly up and grab the star. You could also type "flamethrower", "mech" or "fighter jet" and blow the tree to pieces.
That's the basic idea, the only limitations are dirty words, licensed stuff and nouns only. They have "tens of thousands of words" and the items act like they would in the real world eg. you can chop with a sword, throw a ninja star, a lion will eat a rabbit, a toaster will cook bread etc.
Here are some gameplay videos:
Gametrailers E3 gameplay
And a preview from Joystiq
It will be out this fall.