As we briefly covered in Chapter 4, Selecting the Right MCU, the acronym UART stands for Universal Asynchronous Receiver/Transmitter. UART hardware takes bytes of data and transmits them over a wire by modulating the voltage of a signal line at a predetermined rate:

The asynchronous nature of a UART means no additional clock line is needed to monitor individual bit transitions. Instead, the hardware is set up to transition each bit at a specific frequency (baud rate). The UART hardware also adds some extra framing to the beginning and end of each packet it transmits. Start and stop bits signal the beginning and end of a packet. These bits (along with an optional parity bit) are used by the hardware to help guarantee the validity of packets (which are typically 8 bits long).