WHAT IS AN UART?
An UART (Universal Asyncronous Reciever Transmitter) is situated in the com-ports
in all computers. It is a device that converts parallell data to serial data without a common clock. when sending data between two uarts, uart A starts its clock and puts the TxD (transmit data) line low. When that happens uart B starts its clock too, and a byte is sent. UART A then sends one or two stop bits, that tells UART B to stop.
because of that no common clock is used, the clocks in the UARTs must be very precise. Also, the clock-speed must be 16 times higher than the desired baudrate.