@me.com or @mac.com vs. @icloud.com
This thing of @me.com, @mac.com, and @icloud.com is confusing. Why are there 3 different ones? Can you use more than one? I have a me.com one but now I'm being offered an icloud, com one, and I don't know how to decide which one to use.
OS X Mountain Lion (10.8.2)