The second (and the last so far) article about Master Boot Record and Windows boot process describes what the code stored in this special sector actually does. The article has been moved to English version of this site, so go there and read it

Master Boot Record (MBR) is usually a label for the first sector of a hard drive. The sector contains part of the operating system boot code and information about primary partitions of the hard drive. The boot code gets executed just after BIOS finishes its initialization and performs POST sequence. Main task of the boot […]