This document has been edited to reflect the arrival of OS X 10.9 (Mavericks) and Messages 8.
At OS X 10.8 (Mountain Lion) iChat becomes Messages at version 7.0 ( a continuation of the iChat version Numbering).
We now have Mavericks (OS X 10.9.x)
In Mountain Lion the app can do everything that iChat 5 and 6 can do PLUS it can do iMessages but in Messages 8 (OS X 10.9.x) we lose the ability to do iChat Theatre.
iMessages is the "new" service by Apple to be able to send Messages in text only to iOS and Macs that are using either Messages on an iOS devices or a Mac and it's version.
This needs you to "register" an Apple ID with the iMessages Server.
The iOS device can then have the Apple ID added to the Send and Receive part of the Messages Set up (Settings > Messages > Send and Receive on an iPhone for instance).
On an iPhone this will link the iPhone Number to the Apple ID and iMessages sent to the iPhone at either number or ID will sync to the Messages app on the Mac (And any other iOS devices you set up).
The main Interface window in Messages (Window Menu > Messages CMD +0) can be used to Start new Messages (icon top Left)
Then either Buddies (iChat Style) or Contacts (the Address Book is now called Contacts) can be entered either by the Blue + icon that appears top right or by typing in people's Names in the "To" spot.
The name will show various options if the person is in the Contacts list and has Screen Names, Jabber IDs and iMessages details in their Address Card.
I have typed in "ral" here and it shows some details from several Address Cards (I have second and third Cards with my name relating to IM accounts on other computers).
It also lists some IM service IDs that are not linked to any Address Card.
You will notice it thinks any Phone number is a potential for iMessage, although it only marks tested ones.
It also marks the Accounts that are logged in as Available.
Once a person is in the "To" spot their name becomes a drop down that can be used to swap to "To" as in iMessage to AIM or Jabber or your "From" options.
Which Account, Jabber, AIM or iMessage you are going to be "calling" from is detailed in light grey in the Text entry field at the bottom of the Window.
By Default the iChat style Buddy Lists are not on show.
IF the Messages Menu > Preference > Accounts has the Bonjour account Active or has other AIM or Jabber IDs logged in then the Window Menu > Buddies (CMD +1) item appears.
Using this show a Combined Buddy List by default.
In the Preferences > General Section these can be Unlinked by Unticking the Line about gathering all accounts into one List.
The Window Menu then lists all the Logged in Buddies lists by their Description field in the Accounts pane of the Preferences
If the Buddy Lists are left on Show at quit they will appear automatically next time the App is opened.
Using the Buddy lists such as Double Clicking a Buddy to Text Chat or using the icons to Video or Audio Chat works the same way as they did in iChat.
Screen Sharing works the same way with any Messages account that can Video (Excludes iMessages and any Yahoo Account).
iChat Theatre in Video chats works the same way although the Menu items for it have disappeared. (iChat Theatre was a way of sending or "Sharing" pic, movie files, Keynote/PowerPoint presentations as the Video feed from your end).
The functions remained in Messages 7 but have been removed in Messages 8
Text Chats and even Audio and Video Chats remain service linked (AIM to AIM, Jabber to Jabber, Yahoo to Yahoo and of course iMessage to iMessage)
That is to say that the iMessages service does not become the overriding all encompassing message service.
If you have Yahoo Buddies then you need to set up a Yahoo account in Messages (same for Jabber and AIM Buddies). The Yahoo account has to use an ID registered at Yahoo and not any of the other services that are allowed in other Yahoo capable apps.
It remains that certain Apple Issued IDs are also valid as AIM Screen Names (if the password is kept to 16 characters and below).
This does mean setting them up as the iMessage then in the Add Pane as a AIM Name.
(Applies to IDs ending with the older @mac.com and then with iCloud issued @me.com and @icloud.com IDs)
Hope this Helps.
7:50 pm Saturday; December 21, 2013
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