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.