The iPhone is an amazing piece of technology that sets the bar higher for all phones. There are many bugs on the iPhone but it is on the Bleeding Edge of technology. It can sometime lock up or become unresponsive. The down side is that you cannot take the battery out of the phone. But thankfully Apple thought of the need for a force shutdown.

it works much like a power button on your computer. When holding down the power button for over ten seconds you can force your computer to shutdown. Of course this isn't the preferred method and should only be done if you cannot properly shut down your pc/mac. Same goes for the iPhone. Much better to do the proper shut down or reboot. 

Proper Reboot - To do a reboot on the iPhone push and hold the Sleep/Wake button located on the top of your iPhone. You must hold the button down until you are prompted to "slide to power off". Otherwise you are just locking the screen. 

Reset/Hard Reboot - If your iPhone will not respond to touching the screen then a Hard Reboot is necessary. You will not lose any information like you would doing a restore. Hold down the Home and Sleep/Wake button (locate on the top of your phone) simultaneously until you get the Apple Logo. The reset is now complete and your iPhone should operate normally again.