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Block Special Files
Block devices or block special files are files in Linux that represents block devices.
In Linux, you can only mount block devices.
About reading block devices directly
I wrote in loop devices that you can only mount block devices, and loop devices are block devices created for disk images.
But someone asked a query about mounting a block device as a file, so that he doesn't have to make a disk image with dd for a recovery utility to read from it.
In this case, usually, you would be able to cat a block device to read the contents, and dd does pretty much that. But the block device descriptor file itself is not the same as the block device.
If you do stat on a block device, you will see that it has a size of 0 bytes, and it will be called a "block special file" as opposed to a "regular file" with a size.
rough advaita-analogy The above line gave me an analogy - we say Nirguna Brahman is without any utility - literally Nir-Guna. And I used to have analogies for why that isn't a bad thing. In this example, similarly, a block device has 0 file size. But if you cat a block device, you will get a lot of data. Because while a block device is empty in size, it is what allows you to read the physical block device.