/sbin is like /bin, but for system administrative type commands. /usr holds all the other software that isn’t critical to get the system up and running.

A device file is a special file that’s like a pointer to a piece of actual hardware, like a serial port or a hard drive. /dev also has some non-hardware special files like /dev/zero. When you read from that one, you get an endless stream of zeros. Or /dev/null, that discards any data that’s written to it.