A Meteor is simply a large rock that falls and explodes, damaging anything in the vicinity.
If created high in the air, it will drop, and explode upon reaching the ground, killing everything and everyone on the map, similar to a Nuclear Bomb. Creating it on or close to the ground, however, does nothing.
In Super Scribblenauts, anything that touches it (including water and lava) results in the same effect as dropping it from a great height.
In Scribblenauts Unlimited, instead of destroying everything, it causes heavy damage to objects near its drop zone.