The Picture below is actually taken from IChat 2 but is very similar in both.
The Elements of the ToolBar change very little thoughout the iChat versions. Obviously iChat 1 does not list the Video menu.
The General section deals with the standard presentation and functions such as what to do about the Login when Quitting iChat and Fast User Switching.
Rendezvous has to be allowed in this Accounts pane if not turned on during the Start Up Screens.
The Messages section in iChat 2 allows you to choose Font Style and colour of your Outgoing Text messages and over-ride the Incoming ones.
Alerts is what it say. You can set either System Sounds, iChat's own or certain formats of your sounds in your Home Folder
This line points to the Server Options (Settings.)
In here you can change the port iChat uses on the TCP protocol to login to the AIM Servers. The default is Port 5190 and many below the 1024 threshold will work
It uses the AIM Service as the Text message part but from version 3 onwards can also join Jabber servers including GoogleTalk.
It can send Files and Pics to other iChat Users and in certian circumstances the Pics are displayed in the Chat window.
In later versions it can have add-ons that enhance video features that have then been somewhat included by Apple in Version 4