Wake-on-LAN (WOL) is a technology that has to be supported by the BIOS and network card. Basically, modern PCs are always on as long as they are plugged in using +5V standby power. When the network card receives a WOL packet, it wakes up the computer, and usually performs some kind of task. Generally, the computer boots from a binary downloaded from a TFTP server on a network.
Desktop machine: 2 x Opteron 246, Asus K8N-DL, 2GB PC3200 ECC Reg., XFX GeForce 6600GT, 74gb WD Raptor, 2 x 19\" LCDs, Windows XP x64
Server machine: Intel P4 3.0GHz 2MB EM64T, ECS i865pe, 1GB PC3200, 36gb WD Raptor, Windows Server 2003
Laptop: Dell Inspiron 9100 (Intel P4 3.2GHz 1MB Prescott, i865pe, 512MB PC3200, Mobility Radeon 9700, DVD+R/DL Burner), Windows XP
Linux: P3 450Mhz, 386MB ram, Slackware 10.1 (Running mySQL/Apache)