UART, or Universal Asynchronous Receiver-Transmitter, is a hardware communication protocol used for serial communication between devices. It converts parallel data from a computer or microcontroller into a serial format, allowing for the transmission of data one bit at a time over a single wire. This makes it ideal for connecting devices like microcontrollers, sensors, and computers. In a UART system, data is sent asynchronously, meaning there is no clock signal to synchronize the sender and receiver. Instead, both devices must agree on parameters such as baud rate, data bits, and stop bits beforehand. This simplicity and efficiency make UART a popular choice for many embedded systems and communication applications.