The MBR is located on the first physical sector of your hard drive, and unless you're running stuff like
FreeBSD with a
dangerously dedicated filesystem, it contains the magic bytes
0x55AA at the end, a
partition table and usually some simple code which is loaded onto 0000:7C00h at bootup, after which your
BIOS will do a JMP FAR to the previously mentioned
address. Usually, this code will load another
boot sector which loads your
kernel and
off you go