To expand Niel's answer further.
With Mountain Lion (OS X 10.8) came the release of Messages version 7.
The Numbering continues form iChat (last version was iChat 6 in Lion)
The app does contain all the iChat features such as Buddy Lists, Video chats an Audio Only chats as well as Screen Sharing with the same accounts you used in iChat (AIM and Jabber).
It has a Interface change though.
A main window is a interface fro starting Chats, having Tabbed chats and starting Video Chats (or invoking FaceTime if you are chatting using the iMessages account)
The app also includes the iMessages account.
This is set like the Bonjour one in that it cannot be deleted but unlike the Bonjour one you can remove the Apple ID that is being used to de-activate it.
The iMessages is an account and a service run by Apple that was invented fro the iOS devices first.
You can iMessages to iOS devices using Apple IDs or iPhone Numbers.
Once you have linked and Apple ID to an iPhone number (set your iPhone up with the same Apple ID) then iMessages (Not SMS though) will sync (be displayed) on all devices that are using this Apple ID.
8:47 pm Monday; February 24, 2014
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad