UART: Universal Asynchronous Receiver Transmitter.

Communication is asynchronous, meaning it does not depend on a synchronized clock signal between the two devices. You can set the data transfer speed and format as needed — you basically set an agreed-upon timing between the two devices.

During serial communication, bytes are broken down into bits, transmitted in a sequence, and then re-assembled into bytes once received.

The receiver and transmitter are set to the same baud rate, which takes the place of the clock in telling how the bits are moving and when.  Start and stop bits are placed at the ends of the string of bits (called the frame), which tell the receiver what to expect.

This is a direct form of communication: only 2 wires needed to connect and transfer data from tx pin to rx pin of each device.

UART is used in RFID modules, bluetooth, GPS, (many instances of wireless communications because UART is good for serial communication between devices non-computer devices).