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