Directly Linked Computers
This is where it all began.
Imagine the scene: You have two computers and want to transfer a message from one to the other. Ethernet, WiFi and Bluetooth haven't been invented yet. How do you do it?
We re-invent the telegraph and use Morse code!
Okay, we don't really, but we do something pretty similar.
To make a telegraph work, we would run a piece of wire between two locations and employ someone to convert our message to Morse code, tap away at a little button for a while and then listen for any replies coming back from the person sitting at the other end of the wire.
Now, if we replace our telegraph operator with a computer, the little button with a computer interface capable of making an electrical pulse on the wire (which is exactly what the little button did), the operator's ear with something to detect pulses arriving on the wire and Morse code with binary ones and zeros (actually, we could use Morse code, but since the computer thinks in binary anyway we might as well make use of it) we end up with a way to send and receive information between computers.
We've just invented the serial interface, one of the simplest and most useful interfaces a computer can have (on a modern-day PC, the USB interface is the latest version of one of these).