WebApplication Domain Object Storage

I think this is a very, VERY basic question, but where do most of you actually store your domain objects that your WebApplication views?

Is it usually a singleton object, stored in the class?  Is there ever a reason you would want more than one?  Multiple users get NEW WebApplications, but they are all using the same copy of the underlying object, right?

Just asking because I am worried I am missing something fundamental!

Rob