The Open Network Loader is the second-stage boot environment (after uboot, grub, etc.).
The Open Network Loader's responsibility is to select the appropriate Software Image file and associated persistant storage files when booting the system.
The Open Network Loader can be used interactively to load arbitrary image files over the network or setup to automatically boot image files located on local persistent storage or delivered from automatic-provisioning servers.
The Open Network Loader is based on Buildroot and BusyBox. This decision was made initially based on system flash capacities.
With the introduction of ONIE as the standard installation environment and large block devices available for the system software the need for a small (8-16MB) Loader environment is no longer necessary.
The Open Network loader system will likely be transitioned to a small debian-based system for easy of development, consistency, package portability between the Loader and Open Network environments (which are already Debian based), as well as reducing build time and complexity.