A block device is a special kind of file that corresponds to a device that is only capable of accessing data by blocks. In other words, read, write and seek operations of a block device must occur at block boundaries.
Most hard disk drives (detected and have the necessary drivers) are block devices in the /dev directory. For example, the primary IDE hard disk drive of the first IDE controller is /dev/hda. Note that a block device can also have sub-components that are also block devices. For example, /dev/hda1 refers to the first partition of /dev/hda.
Note that a block device has a zero “file size”. This is because it is an interface to a real hardware device, not a regular file or directory.