If you touch the tabs icon at the upper right of the stock browser, all of your open tabs will be shown. You can then close individual tabs. There's really no such thing as closing the browser unless you want to go to the Manage Apps menu and force stop it, which is usually unnecessary unless you have a zillion tabs open and your system is slowing to a crawl. Alternatively, if you press the Home button to get out of the browser, then long-press the Home button, you can swipe away the browser, which might stop the process, but that's never been quite clear to me.