Lion = OS X 10.7.x and comes with iChat 6
Mountain Lion = OS X 10.8.x and comes with Messages.
Messages is iChat +
The Plus bit is the iMessages Service from the iOS stable.
It is compatible with Messages on the iPhone and iPad.
It takes an Apple ID to "register" with the iMessages service.
However all the "Accounts" you had with AIM or Jabber work just the same in Messages.
By default the Buddy lists are combined (but can be unlinked in the general section of the Preferences) are are not on show at the App Start.
As the Other thread says you get sent to Make your iMessages Registration first before looking at what were iChat Logins.
From your Buddy Lists you can make the same 3 or 4 way Video chats you did in iChat.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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
"Limit the Logs to the Bits above Binary Images." No, Seriously