UART 
Universal Asynchronous Receiver/Transmitter 
An integrated circuit used for transmitting and receiving data via a computer's serial port. Data is transferred from the computer in units of 1 to 8 bytes, with a separate wire for each bit within the byte. The UART receives a byte and converts it to a sequence of voltage changes to represent the 0s and 1s on a single wire. The modem takes the signal on this wire and converts it into sound. At the other end, a modem converts the sound back to voltages, and another UART converts the stream of 0s and 1s back into bytes of data.