While Apple may not allow third-party developer's iPhone applications to run in the background they must be open to run some of its own software, including Safari and Mail among others, does multitask. So, as is the case with a Mac or PC, through a Force Quit for the former and Task Manager for the latter, you may want to or need to shut down Apple's own iPhone apps to make your device more responsive should performance slow down or even freeze.
Rather than a full hard reset, all you need to do is hold down the iPhone's Home button for a six seconds or so writes the iPhone Blog, however. This will open up resources and return you to the Home Screen in far less time than it takes when resetting everything through a hard reset.
As with the Mac, this procedure is referred to as a Force Quit.