It's about two 30-somethings (Winslet and Wilson) who start having an affair after they take their kids to the same playground (Wilson is a "stay-at-home dad"). A third actor has a completely different relationship with children. That relationship is explored too. It's probably the best film I've seen this year but at the same time it profoundly irritated me.
Winslet plays a self-conscious woman who's clearly intelligent, but is self-conscious about how she looks (Wilson doesn't help much in that regard, telling her she isn't beautiful, which some women seem to hear as "you're ugly"). To me that makes no sense. I mean she's not one of those 6 foot tall stick insect model types but I defy anyone to look at her and think there isn't something about her. So it really irritated me when she bleated on about her lack of looks (I had the same response to an Uma Thurman Janeanne Garofillo film made a few years ago).
All that having been said, the film was pretty good, but Winslet won't win the Oscar. It was hardly the most inventive film and I can't really see why she was nominated.