UNIX, and therefore Linux, recognizes two different kinds of devices: random-access block devices (such as disks), and character devices (such as tapes and serial lines), some of which may be serial, and some random-access. Each supported device is represented in the filesystem as a device file. When you read or write a device file, the data comes from or goes to the device it represents. This way no special programs (and no special application programming methodology, such as catching interrupts or polling a serial port) are necessary to access devices; for example, to send a file to the printer, one could just say
and the contents of the file are printed (the file must, of course, be in a form that the printer understands). However, since it is not a good idea to have several people cat their files to the printer at the same time, one usually uses a special program to send the files to be printed (usually lpr(1)). This program makes sure that only one file is being printed at a time, and will automatically send files to the printer as soon as it finishes with the previous file. Something similar is needed for most devices. In fact, one seldom needs to worry about device files at all.
Since devices show up as files in the filesystem (in the /dev directory), it is easy to see just what device files exist, using ls(1) or another suitable command. In the output of ls -l, the first column contains the type of the file and its permissions. For example, inspecting a serial device gives on my system
The first character in the first column, i.e., `c' in crw-rw-rw- above, tells an informed user the type of the file, in this case a character device. For ordinary files, the first character is `-', for directories it is `d', and for block devices `b'; see the ls(1) man page for further information.
Note that usually all device files exist even though the device itself might be not be installed. So just because you have a file /dev/sda, it doesn't mean that you really do have an SCSI hard disk. Having all the device files makes the installation programs simpler, and makes it easier to add new hardware (there is no need to find out the correct parameters for and create the device files for the new device).