On IBM compatible computers (PCs) running a Microsoft operating system such as Windows or MS-DOS, the hard drive is the C: drive because it is the first available drive letter for hard drives. The computer assigns the A: and B: drives to floppy disks and sometimes other removable media such as tape drives. As you install other hard drives, create new partitions, and add other drives such as CD-ROM or DVD drives, they will be assigned to other drive letters after C, such as D, E, F, G, etc.
For example, most computers today come with a hard drive, and disc drive, such as a CD-ROM, CD-R, or DVD drive. With this common configuration, your hard drive would be assigned to the C: drive, and your CD or DVD drive would be assigned to the D: drive. If no floppy drive comes with the computer because A: and B: drive letters are reserved for floppy drives your hard drive will still be assigned as the C: drive.