The main UUID for an iMessage account is your Apple ID email address, but each account on iMessage can be hooked up to multiple email addresses also. For example if your contact for someone has a phone number and email addresses it will check for the phone number, then check for the associated email addresses. On my mac I receive all my iMessages because I set my reply to address to be one of those email addresses on the phone/pad. And yes, you can iMessage someone to their phone number form your computer / iPad.

iMessage supports all of the end-to-end messaging features above, but doesn't support letting devs use the messaging function other than being able to send one-off messages with attached photos / movies in iOS. Apple has something more like Xbox Live for doing in-app voice chat / messages (GameKit).