Also known as: UART, serial port
A UART (Universal Asynchronous Receiver/Transmitter) is a hardware block that sends and receives serial data over two wires without a shared clock.1
Overview
Instead of a clock line, both ends agree on a baud rate and frame each byte with start and stop bits, so the receiver can recover timing from the data itself. The link uses just transmit (TX) and receive (RX), making it the simplest of the common serial interfaces. A microcontroller usually has several UARTs; one is often wired to a USB-serial adapter to provide a console. Classic RS-232 ports are UARTs with higher voltage line drivers.
Where it fits
UART is the workhorse point-to-point link: a debug/console port, a connection to a GPS or radio module, or a board-to-board serial line. Unlike the multi-drop I²C and SPI buses, a UART connects exactly two endpoints, which keeps it dead simple. Many bootloaders accept new firmware over UART, and it is a fixture of almost every embedded system.