I would say that the major differences are:

Romney and Gingrich are both plutocrats, though Gingrich is more tempered by some populism. Basically, they seem to want to aggregate more money in the already-rich and in corporations.

Santorum is a theocrat. He wants to legislate his personal moral and ethical code.

Paul is a Libertarian with a few socially conservative planks. By and large, he wants the federal government to legislate, enforce, and, in general, do as little as possible.

That's really all of them that have any vague possibility of winning.

Obviously, these are based in part on my opinions. That said, the basic facts should be accurate, even if the way I present them is different from how others might.