Hi,
Yes the fact that Apple call the Service iMessages and the text it send also as iMessages whilst now calling the app just Messages has confused things, particularly when what you want to do is continue iChatting.
A Brief History of iChat (to misquote Hawkins)
From iChat 1 we have iChat as an AIM client.
iChat 2 adds AV abilities
iChat 3 adds Jabber. (this add Google with a bit of extra work)
iChat 4 sorts adding Google as a separate option. (and also 4 way Video chats and effects)
At iChat 6 the option to add Yahoo account is added but these don't invoke the iChat Video (and Audio Only) chats.
Messages beta (version 6.1) was test run in Lion until December 14th 2012.
It added the iMessages account to what was iChat and changed the interface a fair bit (And changed the name although the Version numbers continue.
The iMessages account does not video iteslf but Invoke FaceTime rather that using the iChat Video abilities that are still there.
Messages 7.0 is in Moutian Lion with OS X 10.8.2 updating it to version 7.0.1
Messages 8 comes with Mavericks and at the time of writing has not received any updates.
To restate.
Messages will do iChat style Video (And Audio Only chats) with AIM and Jabber accounts.
It will also do them for LAN based Bonjour Connections.
Whilst using a Buddy list the Yahoo account does not do the Video that iChat did.
The iMessages Account is Text only but will use FaceTime to "convert" an iMessages Session to Video in FaceTime.
In addition to anything said so far the iMessage account can be "synced" to your iOS devices as well.
(Sync means "Display on all devices" in this case - anything you do to the iMessage after that is on that device only)

8:06 pm Sunday; January 5, 2014
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