What kinds of things can you diagnose with a compression tester and a vacuum gauge? And exactly how do you use them to perform these tests?

I'm interested in checking the engine in my 450, and hopefully pinponting things such as worn rings, blown head gaskets, and so on.

Hopefully, I'd like to test this Mercedes' vacuum door lock system as well.

I've hearn terms such as "leak-down test," "wet test/dry test," and so on, but at this point, it's all Greek to me!

Any and all help appreciated