Block devices are storage hardware (disks, SSDs, cards) exposed as files under /dev/, named by how they connect — sd* for SATA/USB, vd* for VMs, nvme* for NVMe.

A block device moves data in fixed-size chunks (blocks), as opposed to a character device (like a keyboard) that streams byte by byte. Linux represents every disk as a special file in /dev/, so the kernel can talk to wildly different hardware through one uniform interface — and so you can manage it with ordinary tools once a filesystem is mounted on it.

The name encodes the transport, which is why it tells you something about the hardware:

The trailing letter counts whole disks (sda, sdb, …); a trailing number is a partition on that disk (sda1, sda2). NVMe is the odd one out: nvme0n1 means controller 0, namespace 1, and its partitions get a p: nvme0n1p2.

Why it matters: a /dev/sdX letter is assigned in detection order, so it can change between boots if you add or remove a disk. That is exactly why filesystems are mounted by stable UUID rather than by device name (see the UUID card).

Go deeper:

• Device file (Wikipedia) — the block vs character device distinction (buffered vs unbuffered) and the /dev model.