In iChat everyone you chat to is a Buddy.
AIM, Jabber or in iChat 6 Yahoo as well. (The app can also do "Bonjour" to any LAN based Mac that is also running iChat or Messages with Bonjour enabled).
That is to say that the app can join these Messaging services if you have a Valid ID that will work with them.
Messages app in Mountain Lion.
Adds iMessages to the above as an Account but does not run a Buddy list for this Account and uses your Contacts App List (formerly the Address Book)
The iMessages account can use your Apple ID that your "register" with the service or the iPhone Number to send Messages from.
Whilst the iOS version can somehow "tell" when a number is not registered with iMessages and will send SMS instead the Mac version cannot.
However in iChat you could use your AIM Buddy list to add a Phone Number - as if it were a Buddy - to the list, as in my pic ( I moused over it to bring up the details).
So I was trying to make the Distiction between people you chat to in a Buddy list as you would in iChat and those people you can message using the iMessages Services and your Contacts List.
AIM restricts who SMS Messaging works for.
1) To the USA
2) to specific Mobile/Cell Phone companies (Carriers)
Accounts in the Preferences and Main Messages Window.
Window Menu and a single Buddy list
By Default the Window Menu displays a Combined Buddy List (AIM accounts mixed with Bonjour, Jabber and Yahoo ones).
They can be separate like this when the line about Gathering the accounts is Unticked in the General Section of the Preferences.
Double clicking a line in a Buddy List opens a Text Chat with the "Buddy".
In Messages the Main Messages window is also a tabbed Chat window.
AIM Names can be registered at AIM
However IDs issued by Apple ending in @mac.com, @me.com and @iCloud.com are also Valid AIM Screen Names.
(in some cases this may mean registering an Apple ID for iMessages and then using the + button below the List to add it as an "AIM" Name - Although possible I would tend not to do it or at least not with one linked to an iTunes account.)
So, Buddies are people that you may have connected to in iChat via AIM, Jabber or Yahoo.
Contacts are used direct from the Contacts App list for the iMessages contacts you have.
If you have an AIM valid Screen Name it can be used in Messages and then you can add certain qualifying Phone Numbers As Buddies to your Buddy List and send them Messages.
Hope that helps.
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