I'm not exactly sure how to describe it to you. You could technically call it a “situational comedy,” but calling it a SitCom is a bit misleading as it's not the kind you'll find on regular TV. The main character in the show is Larry David, co-creature of Seinfeld. It revolves around him and his (fictional) wife and their life together.
So, he is a Jewish man living in California with his lovely wife. He has a manager he sees frequently and various celebrity friends like Richard Lewis, Ted Danson (from Cheers), and cast from Seinfeld including Julia-Louis Dreyfus. These people apparently play “fictionalized” versions of themselves.
Larry tends to find him self knee-deep in life's little problems. Many of these he creates for himself by saying or doing something that's not quite socially acceptable. There are really plenty of examples to give, but I don't want to give away too many spoilers. The first episode opens up with him talking to his wife about the fabric “tent” produced by his pants when he sits down. This later turns into an awkward situation when his wife's friend accuses him of getting an erection.
The topics covered in the show are clearly for an adult audience, hence it being on HBO. There are adult situations and language, which provide some more flare. The wife of Larry's manager definitely has a mouth on her and drops some serious F-bombs, but not just for flavor, it's because she yells at Larry and his manager for doing or saying stupid things.
It's a very funny show, I think. There is no laugh track, which I think is great because the audience doesn't need to be told when to laugh. It is funny, but often, I find myself feeling sorry for Larry. He can be a bit of an ass and so are other characters. One interesting fact about the show is that it is often unscripted or loosely improvised.
I think the sad thing is that I can sometimes relate to Larry. Sometimes I find myself saying or doing something completely stupid that comes back to bite me. It's tough saying or doing things “off the cuff” as they say. These things happen to some of us, which creates social anxiety because then you have to be careful what you say or what you do. I personally find it a difficult thing to manage.
Anyway, I do seriously recommend this show. If you're interested in what I've described, check it out or feel free to ask me for more information. I'll be glad to provide more examples. I think it's very interesting and humorous.