A partition on a computer hard drive is a section capable of storing the machine's operating system (OS), user files or both. You can have up to four primary partitions, also known as "volumes," on a hard drive. Changing them is risky and can make files inaccessible or render your computer inoperable.
You must assign a drive letter to each partition in a Windows computer system, such as "C." After partitioning a disk, you must format each partition with an OS-appropriate file system before you can do anything with it. For example, if you're installing Windows 7, the New Technology File System (NTFS) is your best choice. For older operating systems, such as Windows 95, select the File Allocation Table (FAT).
During OS installation, Windows creates one primary partition that encompasses the entire disk. However, you may create up to four primary partitions. If you want more than one OS on your computer--known as a multi-boot system--you must create one primary partition for each OS. You can also use additional partitions to organize, separate and store data files. Doing so often improves system performance, and it can come in handy if the OS ever becomes corrupted, unbootable or infected with viruses.
Whichever primary partition from which you load your OS at boot time becomes the active partition. You can designate only one partition active at a time. If you have a multi-boot system, you already know this fact indirectly, as switching from one OS to another requires that you reboot the computer. When you make your OS selection at boot time, you mark that specific operating system's partition as active.
If you're only running one OS, your "C" partition, or "C" drive, should remain permanently active. If you accidentally mark a partition as active that doesn't contain, your system won't boot. Therefore, the "C" partition in a single-boot system is both primary and active. In a multi-boot system, the active partition changes when you load another OS.