A small program that runs as a BIOS extension, usually on an EPROM on the NIC. It handles the BOOTP query and TFTP loading and then transfers control to the loaded image.
It uses TCP/IP protocols but the loaded image doesn't have to be Linux. The loaded image can be anything, even DOG.
There are two free implementations of TCP/IP net loaders: Etherboot and Netboot: Etherboot uses built-in drivers while Netboot uses Packet drivers.
They can also be loaded from a floppy for testing and for temporary setups.