Hi,
iChat became Messages at Mountain Lion.
It has all the iChat functions except for iChat Theatre unfortunately.
It adds a New Service, iMessages which sort of becomes the "lead Account" based on the fact it will ask you for an Apple ID to "Register " for iMessages when you launch it for the first time.
System Preferences > Internet Accounts may also list older "iChat" IDs and have enabled them. (you can add them in either the app of here)
The iMessages bit works with iOS devices that are using iMessages from the iOS Messages app.
This means you can link your iPhone Number to the Apple ID and then iMessages will "Sync" (Display on all devices).
By Default the Buddy List are also not on Show and they are also initially combined into one list.
The can be displayed in the Window Menu (> Buddies) and separated in Preferences > General Section.
Then the Menu looks like this.
As the Buddy Lists are not on show there is a new tabbed chat and interface window to Start Chats and iMessages conversations from.
AIM and Jabber account will still do the iChat style in app Video.
The Yahoo Account does not do Video (never has done).
The iMessages account invokes FaceTime once you have Started an iMessages conversation and then use the Video icon on the Main Window. (See the yellow bits in the pic above)
7:53 pm Thursday; April 17, 2014
iMac 2.5Ghz i5 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