An RS-232 interface has the following characteristics:
You probably already saw this connector on the back of your PC.
It has 9 pins, but the 3 important ones are:
Using just 3 wires, you can send and receive data.
Data is commonly sent by chunks of 8 bits (we call that a byte) and is “serialized”: the LSB (data bit 0) is sent first, then bit 1, … and the MSB (bit 7) last.