Hi,
iChat is the forerunner to Messages.
At version 1 it was an AIM client app (you used an Apple issued ID as a valid AIM Screen Name or an actual AIM registered Name) and could Text Messages to other Mac users using iChat or AIM for Mac and to AIM for PC users.
It also had the ability to find other Macs on you LAN and chat to them, iChat to iChat using what was called the Rendezvous account (Later Bonjour)
iChat 2 added the Video capabilities.
You could Video or Audio Only chat using the AIM Account to PC AIM users to to iChat users that had iChat 2
The Bonjour account could also do the same to LAN based Macs.
iChat 3 added Jabber abilities.
You could do the same Video and Audio Only chats to iChat users using Jabber based IDs (which included Google Talk IDs with a small amount of work to adjust the server name)
iChat 4 adds the ability to add a Google Account without extra work.
It also adds multiple chatting in Audio (10 people in total) and Video (4 people in total)
At iChat 6 in Lion (OS X 10.7) the Yahoo Option is added but this is text only (it does not do Video/Audio even Mac to Mac).
Messages
Messages starts in Mountain Lion but is numbered onwards from iChat at Version 7
It changes the main interface window ( a chat window with bits that access the Contacts List to use with the iMessages account and the Buddy list info without displaying the Buddy Lists)
It also add the iMessages account.
iMessages
This is a service run by Apple for iOS devices (iPads, iPhones and some iPod Touch devices) and on Macs from Mountain Lion (OS X 10.8) onwards.
At OS X 10.8.2 Messages is updated so it "syncs" with iPhones and iPads.
This links the iPhone Number to the Apple ID and it then displays the iMessages sent to the iPhone Number on devices also using the Apple ID and Vice Versa.
The Sync as such is only "Display on all Devices" and anything you do to an iMessages on one device is not copied across to other devices.
You can use any Apple ID be it an email issued by Apple that is still valid (Certain @mac.com IDs and iCloud linked @me.com and @icloud.com IDs) or any other email you created your Apple ID with.
Messages can display the Buddy Lists and also separate them (The default is to merge them) so that you can use the app like iChat.
The "iChat features" that were there before are still there - AIM to AIM Video, Audio Only and Screen Sharing Mac to Mac. The same for Jabber accounts (GoogleTalk and Facebook Chat are also Jabber servers) the Text Only Yahoo account and the Bonjour account that can do the same as AIM and Jabber logins.
In Messages the Video icon is viewable in the main Chat window when doing Jabber, AIM, Bonjour AND iMessages chats.
Using it in the Jabber, AIM and Bonjour chat will use the "iChat Side" ability to video or Audio Chat or Screen Share within the App (like iChat)
However the iMessages account will invoke the FaceTime App to do video and this will be only 1-1
Summary.
Short version
Messages is an App that adds the iMessages account to what was the iChat application which effectively means it can join five services.
AIM
Jabber (including Facebook Chat and GoogleTalk amongst others)
Yahoo (Text only)
Bonjour (Apple Only Mac to Mac Chats (full features) to LAN based machines.
iMessages (Text only to Apple Only Devices (running iOS or Mac OS X), although it will invoke FaceTime for Video chats.
iChat is an app in OS X 10.7 and earlier (Lion back to Panther effectively) that at the iChat 6 version (Lion) could Video chat AIM to AIM or Jabber to Jabber to other Macs. It could do the same in Audio Only Chats. In Later versions these could be multiple way chats.
9:55 pm Sunday; August 24, 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
