The /dev directory contains entries for the physical devices that may or may not be present in the hardware.  The hard drive partitions containing the mounted filesystem(s) have entries in /dev, as a simple df shows.
bash$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda6 495876 222748 247527 48% / /dev/hda1 50755 3887 44248 9% /boot /dev/hda8 367013 13262 334803 4% /home /dev/hda5 1714416 1123624 503704 70% /usr
Among other things, the /dev directory also contains loopback devices, such as /dev/loop0. A loopback device is a gimmick that allows an ordinary file to be accessed as if it were a block device.  This enables mounting an entire filesystem within a single large file. See Example 13-6 and Example 13-5.
The entries in /dev provide mount points for physical and virtual devices. These entries use very little drive space.
Some devices, such as /dev/null, /dev/zero, and /dev/urandom are virtual. They are not actual physical devices and exist only in software.
A block device reads and/or writes data in chunks, or blocks, in contrast to a character device, which acesses data in character units. Examples of block devices are a hard drive and CD ROM drive. An example of a character device is a keyboard.