An Acronym for Pre-boot Execution Environment.
If you have a PXE-bootable card, and a compliant motherboard BIOS, it will boot off PXE just fine. You can see it trying to do this as the machine boots – it might prompt to boot off the NIC, or it might say something about DHCP, or so on.
The next step, of course, is to making your PXE-booting machine do something… PXELINUX, which is part of SysLinux, will help you out here. PXES (mentioned on the DisklessWorkstationNotes page) also makes use of PXE.