Block devices are a specific type of Unix device node, typically representing storage devices such as hard drives, CD-ROMs, etc.
Block devices are a specific type of Unix device node which the operating system reads from and/or writes to in blocks of data -- typically a multiple of 512 bytes.
Some common block devices are hard drives and CD-ROM systems.
Traditionally block devices have been cached by the operating system. As this caching can lead to undesirable behavior a "raw" block device is typically also provided.