All over and allover both mean “everywhere” but are used differently. All over is an adverbial phrase:
The children ran all over the yard.
However, all over is often used less literally, just to mean “a lot”:
There were professors arguing all over the conference room.
Allover usually acts as an adjective:
I prefer a rug with an allover pattern, so you can spill coffee on it.
Additionally, allover can also be used as a noun, referring to a kind of lace or embroidery with an allover pattern, but of course this is an extremely rare usage. Even as an adjective, allover is not commonly used.