The Preboot Execution Environment (PXE) is an environment to bootstrap computers using a network card (i.e Ethernet, Token-Ring) independently of available data storage devices (like hard disks) or installed operating systems.
Preboot_Execution_Environment at Wikipedia.
PXE Boot Menus
You can create a graphical boot menu to allow selection from multiple boot options. This is documented here.
Note: if running Lenny read menu.txt.gz in /usr/share/doc/syslinux. In addition to pxelinux.0, you must also copy /usr/lib/syslinux/menu.c32 into /var/lib/tftpboot and add DEFAULT menu.c32 to top of menu. This is a recent change as of Syslinux 3.31 and not shown in most examples on the net (and not even in an April 2008 Linux Journal story on PXE menus.)