precaution: do not take my statement word by word, this is just very very rough and inaccurate guide on this topic.
--- it all starts with a cold reset.
--- the processor will try to look for a BIOS rom at a pre-defined sytem IO address (F000:FFF0)
--- if the ROM is found and it is valid, processor will try to execute the BIOS code in the rom, and doing all hardware validation and initialization stuff (so called Power On Self Check, or POST)
[ after the memory controller is initialized successfully, part of the ROM code and data will be copied (or you call shadowed) to the DRAM, and execution continues from there.]
--- when all this done, and a default or user preferred fix storage is detected (e.g. a hdd)
--- processor will look into the first sector of the hdd partition (so called Master boot record, or MBR)
[MBR is a 512 bytes long area in hdd, and it contains the disk partition, vendor info, and something related to 2nd stage boot loader, while the MBR is considered the 1st stage boot loader]
--- MBR machine code may boot directly to OS or it may pass control to 2nd stage boot loader (normally, it refers to famous names: LILO, GRUB or window NTLDR)
[these 2nd stage boot loader normally have some user interaction shell, and allows user to boot from one of several installed OSes]
