Universal Asynchronous Receiver/Transmitter (UART) is a serial communication protocol that has been around for a very long time and in common use before the prevalence of USB. UART actually refers to the electronic hardware used to implement the serial protocol, although it can be implemented in pure software.
Today, SPI or I2C tend to be used in preference to UART. GPS receivers are a common example where serial communication still prevails. If you have ever connected an Arduino to a PC for flashing or debugging, it's a serial communication protocol that the devices are using, with UART hardware being present in the Arduino.
We have now learned many of the standard ways that we can use to interface electronics with our Raspberry Pi, including analog and digital electronics, PWM, wire protocols such as I2C and SPI, and serial communication. We will start to see many of these interfacing options in practice and get a feel for...