The Raspberry Pi doesn't come packed with any buttons for power or resetting. Raspi.TV noticed, however, that the newest Rev 2 version of the Raspberry Pi actually comes with a place to add in your own reset switch.
Generally, if you want to reset the Raspberry Pi, you have to unplug it and then plug it back in, or enter a command with a keyboard. That's fine if you actually have a monitor hooked up, but if you don't, constantly unplugging it is going to wear down the connections. To add a a reset switch, you'll just need to solder on a couple of pins as described in the video above. You can then use any old PC reset switch, or just join the pins with a jumper when you have to reset it.
The Raspberry Pi isn't exactly made for frequent resets, but it comes in handy on occasion. Head over to Raspi.TV for the full guide.