This is my understaning..

by default Nhibernate implements caching,eg i ask nhibernate to return me a collection of customers during an Isession.Then,when next time i need this collection,say from a different page,nhibernate gets from cache,not database..tite?

When Isession gets destroyed? After each request from a user,or when i say Isession.close,or when users session gets over,say when he closes the browser..

And,how is second level of caching different?

thks

First level cache lives with the ISession. When you close ISession it goes away. Second level cache can live longer.

ISession is destroyed with GC as any other object. But it is required to .Close it as fast as possible to free unmanaged resources (eg. Database Connection).

I recommend "Hibernate in action" book - it will answer all such a questions and allow to avoid many traps with NH, take a look if possible.