Formats

When we create a regular Linux program, we generate an ELF file, which is read by the OS.

Without an OS, we can use the following formats:

boot sector, of which MBR is an important type.
El Torito for CDs: https://en.wikipedia.org/wiki/El_Torito_%28CD-ROM_standard%29
multiboot
hybrid boot sector / El Torito. It is possible to generate images that can be burnt either to USBs or optic disks.

The Linux kernel 4.2 for example does that by default upon make isoimage.
PXE: https://en.wikipedia.org/wiki/Preboot_Execution_Environment and its implementation https://en.wikipedia.org/wiki/IPXE

Boot from the network. TODO how does it work exactly? I suppose there is a server, and then the BIOS can download the boot sector from it.