The best explanation I've found for a postBack is from Wiki.
a postback is an HTTP POST to the same page that the form is on.
While the article does explain how a second page was needed in ASP, but no longer needed in ASP.NET, it doesn't give much detail or background. I am looking for a freakin' tome of information on PostBacks. Much like the simple question of "how can I clean a house" can be addressed by this 900 page book. I don't need 900 pages worth, but details please. I found a nice little tutorial for ASP.NET life cycle, but it seriously glosses over postbacks (amongst other things).
I am looking to the developers who have been around before .NET and really don't take these kinds of things for granted. Books and hyperlinks are reasonable answers or additions to your answer.