New to the mac world,but really loving it.Should have done it years and years ago.....(why would anyone use a pc now?)

So,loving pretty much all of it but....
1)What is ichat?From what i can fiure,it enables people to talk that have an AIM,Bonjour account?
2)My ichat starts when i boot up and my name appears at the top of the "bonjour"window.I'm guessing i must have registered when i first got this mac months ago...I certainly didn't register with a "AIM" or any other account....when i click on the logo on the dock the bonjour box opens but there are no "buddies"in it.and when i go to "buddies" at the top the options are all in pale grey indicating they are not available..How do i "unregister" my name(should i?)
3)I now have set up an AIM account but the ichat stays in bonjour mode..

Its really the only frustrating thing on the comp......

thanks for any assistance.
b.