Ok, I'll admit, I've only seen the movie once and that was on video. So this is going to be short.
The main character in this film is Paul Reiser's Modell. Very simply, this is a movie about possibilities. All the variations we can imagine of Modell. Why Modell? He's the pure form of this uber-character. Like a lot of these essays, this idea started with an itch, and that itch was the realization that Modell only appears at the Diner. He has no life that we see other than meeting his friends, the characters with whom we're spending all our time. So Modell is the pure form of this character, and then we get all the variations: the rich one, the poor one, the one who's married (and we never really see the wife, because she's not a variation on Modell), the one who's getting married, the one who went to college, the female version, and so on. I forget the details, I saw it a long time ago. But I think you get the idea.