All computers have some method of loading an operating system, called booting it. The drive or device containing the operating system must be made bootable so the BIOS can understand that it contains the operating system's bootloader. This may be done by setting the partition "active", placing certain files in special folders (for example,
COMMAND.COM in the root of the drive for MS-DOS, placing System and Finder in the system folder for Macintoshes), or performing other configurations.
As usual, Wikipedia has an article on the process. For more modern computers, see this section http://en.wikipedia.org/wiki/Booting#Modern_boot_loaders