Hi,
The Messages app can join several of the well known messaging services.
Previously the app was called iChat (version 1 through 6) and initially started as an AIM capable app.
Since version 1 the app has also had the Bonjour Option.
This does all the same chat (Text, Audio only, Video and Screen Sharing) as the original version 2.1 which introduced Video.
At iChat 3 Jabber abilities were added.
At that Time Google started GoogleTalk based on using Google Mail IDs using a Jabber server.
Google have never stuck to the Jabber server naming protocol as everyone else (mainly as there are both @googlermail.com and @gmail.com IDs)
At iChat 4 Apple got around this by adding a separate mechanism for adding a Google account but it still Jabber behind that.
In the early versions of ichat it was expected that you had one AIM account and one Jabber account.
This meant that in the Preferences > Accounts the accounts were called AIM and Jabber and Bonjour which is then repeated in the Window Menu.
If you added more accounts then the Description field in the preferences comes into play.
iChat 6 add a Text only Yahoo ability.
Messages 7.0 (Mountain Lion) add the iMessages account which is Apple devices only. It is Text Only and comes with a fairly major interface change for the app.
The Main window can be used to access Contacts (app) to connect to people you know via iMessages and their Apple ID or iPhone number.
It also allows you to access your Buddies (AIM, Jabber or Yahoo and Bonjour) without displaying the Buddy lists (this can still be done though).
Messages 8 comes with Mavericks and has had 3 updates up to and including OS X 10.9.4
Basically the Description Field then names the Account in the Account's list.
It labels the Buddy List window when Displayed.
It also appears as that name in the Window Menu when you chose to show Buddy lists separately
AND it is displayed where you show "Jabber" in your pic.
So to clarify so more.
As show in my Pic you can have many accounts.
If your have only one of each type they default to being shown by Type Name.
If you do not enter a Description but have two Jabber accounts for instance then they will revert to Account ID as the name (similar to the partial shown as item 8 in the Window Menu - although one does happen to be a AIM valid Screen Name).
However changing the Description (red in the pic below) shows in the List (blue) and in the wind Menu (Green) and in the bottom left corner of the main Messages window (not shown)
Hope this helps
7:13 pm Sunday; August 3, 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