From MEPIS Documentation Wiki
The /dev subdirectory contains special files which link to the various input/output devices on the system. Because access to these is required at boot time, this is a subdirectory of the root filesystem.
The /dev directory is where all of the hardware (and sometimes software) devices are stored. In Linux everything (including devices) is treated like a file. Devices are read from and written to just like any other file. This should not be done by regular users as the files are in a raw state. The average user will not have permission to do anything to any files in the /dev directory. Take your hard drives for example. Most computers have two IDE channels and each channel can have two drives on it. Each channel has a master drive and a slave drive. The master drive on channel one would be /dev/hda, slave on channel one would be /dev/hdb, master drive on the second channel is /dev/hdc and the slave drive on the second channel is /dev/hdd. There is a file in /dev that corresponds with every piece of hardware on your computer. more detail