Device file system

A device file system or special file system allows software to interact with the device driver using standard input/output system calls, which simplifies many tasks.

It includes device files, device nodes', or device special files which are interfaces for a device driver that appears in a file system as if it were an ordinary file. This allows software to interact with the device driver using standard input/output system calls, which simplifies many tasks. There are also special device files on Microsoft operating systems, such as MS-DOS and Windows.

Device files often provide simple interfaces to peripheral devices, such as printers. But they can also be used to access specific resources on those devices, such as disk partitions. Finally, device files are useful for accessing system resources that have no connection with any actual device such as data sinks and random number generators.

MS-DOS borrowed the concept of special files from Unix, but renamed them device files. Because early versions of MS-DOS did not support a directory hierarchy, device files were distinguished from regular files by making their names reserved words. This means that certain file names are reserved for device files, and cannot be used to name new files or directories. The reserved names themselves are chosen to be compatible with "special files" handling of PIP command in CP/M.

There are two general kinds of device nodes in Unix-like operating systems, known as character special files and block special files. The difference between them lies in how data written to them and read from them is processed by the operating system and hardware. These together can be called device special files in contrast to named pipes, which are not connected to a device but are not ordinary files either.

Reference:
http://en.wikipedia.org/wiki/Devfs