MS-DOS recognizes two types of device: character and block. A character device performs input and output a single character at a time. Examples are the keyboard, screen, serial port, and parallel port. A block device performs input and output in structured pieces called blocks. Block devices include all disk drives and other mass-storage devices on the computer.
A device driver supports either a character device or a block device, but never both. The type of device a driver supports determines both the functions the driver implements and the information the driver supplies in its device-driver header and to MS-DOS when the driver is initialized.