As a general rule, apps don't close, even when you switch from one to another. Everything stays open and (for want of a better description) minimised. Everything should be as you left it when you return, although Safari may sometimes need to reload the page you were viewing when you left it.
The only ways to close apps are to press and hold the Home button until it returns to the Home page, or power off and restart. IN my experience this is only really necessary if you are abroad and you want to make sure that your apps are not running up data roaming charges without your knowledge.