How do I restart my iPhone and why should I? Often people talk about restarting an iPhone in the same way you restart a computer.

How do you do this, what buttons do you press?

Also how/why is this different to just turning off the iPhone using top button and then turning it back on?
You can restart the iPhone by holding the power button and home button for about 5 seconds (until you see the apple logo pop onto the screen). You may have to do this if the phone freezes, or otherwise becomes unresponsive.
http://www.apple.com/support/iphone/assistant/phone/#section_2