you use the WOL feature of your network card to wake up your computer. all you need to do is enable WOL in BIOS and know the MAC address of your NIC and send a magic packet to it with the MAC. this can be done remotely. a good use for it is if you need to turn on your comp from a remote location to use terminal services or something similiar like remote destop or PCA.
you can turn on your friend's comp using the internet and make it download a bunch of pr0n.