Good tutorial, explains in a really simply manner how to write the initial instructions of a boot loader.
We'll eschew the discussion of the intricacies of how the BIOS works, but here's what you need to know. When you turn your computer on, the processor immediately looks at physical address 0xFFFFFFF0 for the BIOS code, which is generally on some read-only piece of ROM somewhere in your computer. The BIOS then POSTs, and searches for acceptable boot media. The BIOS accepts some medium as an acceptable boot device if its boot sector, the first 512 bytes of the disk are readable and end in the exact bytes 0x55AA, which constitutes the boot signature for the medium. If the BIOS deems some drive bootable, then it loads the first 512 bytes of the drive into memory address 0x007C00, and transfers program control to this address with a jump instruction to the processor.