(simplifying slightly) You can have as many physical drives as your machine supports. Each physical disk can have a single "Active Partition". When you boot your machine, it will boot whatever boot loader is on the active partition of the primary drive.
A boot manager is nearly always installed with an OS (for example, NTLDR on Windows XP and below, BOOTMGR on Windows Vista and above, GRUB on Linux).
Most of these are configurable and you can simply tell it to boot from another partition.
If you have Windows and just want to experiment with adding the ability to boot from other partitions, I recommend you take a look at EasyBCD (download non-commercial/free edition).
If you have any more specific questions, please ask and I'll be happy to modify the answer.