Wheels size and how are they read.

I hope this isnt too newbish of a question. Up until now all ive done is buy rims . I haven't really looked at the different's in them Im looking at rims yet again but have seen that its not just the size anymore but another number after that. for example. 18 x 7.5 or 18 x 5 does the rear number mean wheel with. And if it does do you put bigger wheel with on the front or rear. like a said a bit newbish but hey. If you dont ask. you'll never know.

Thanks