Here is a decent to-the-point comparison I found for you:
As you work up through the version, each one can support more ram, cpus, virtual sessions, cluster services, hyper-v, and features like hot swap cpu's.
To my knowledge, they are not licences by cpu's. Each version however supports a certain amount of cpu cores and supports a certain amount of "virtual" stations. The license you buy is for the type of server you are running it on. Beyond that, it all CAL (user/device based licensing).