The MBR is the first 512 bytes of your hard drive.  All x86 systems that don’t use EFI start booting from this sector.

There is a lot of information on the web about it already, but I am working on a program that will decode the MBR and display its secrets for you to see.  It is entirely written in JavaScript, PHP and HTML (with a little CSS for flavor), and it should be ready to post in a few weeks.