It is customary for authors explaining device drivers to start with a complete explanation of character devices, saving block device drivers for a later chapter. To explain why this is, I need to briefly introduce character devices as well. To do that, I’ll give a little history.

When Unix was written 25 years ago, its design was eclectic. One unusual design feature was that every physical device connected to the computer was represented as a file. This was a bold decision, because many devices are very different from one another, especially at first glance. Why use the same interface to talk to a printer as to talk to a disk drive?

The short answer is that while the devices are very much different, they can be thought of as having most of the same characteristics as files. The entire system is then kept smaller and simpler by only using one interface with a few extensions.
A practical effect of the difference is that filesystems can only be mounted on block devices, not on character ones. For example, most tapes are character devices. It is possible to copy the contents of a raw, quiescent (unmounted and not being modified) filesystem to a tape, but you will not be able to mount the tape, even though it contains the same information as the disk.

Most textbooks and tutorials start by explaining character devices, the sequential-access ones, because a minimal character device driver is easier to write than a minimal block device driver. My own Linux Kernel Hackers’ Guide (the KHG) is written the same way.