devfs is a specific implementation of a device file system on Unix-like operating systems, used for presenting device files. The underlying mechanism of implementation may vary, depending on the OS.
Maintaining these special files on a physically implemented file system (i.e. harddrive) is inconvenient, and as it needs kernel assistance anyway, the idea arose of a special-purpose logical file system that is not physically stored.
Also defining when devices are ready to appear is not entirely trivial. The 'devfs' approach is for the device driver to request creation and deletion of 'devfs' entries related to the devices it enables and disables.
Read more about this topic: Device File