UART & USART - What's the difference
UART = Universal Asynchronous Receiver Transmitter

USART = Universal Synchronous Asynchronous Receiver Transmitter

A USART can act in Asynchronous mode just like a UART. But is has the added capability of acting Synchronously. This means that the data is clocked. The clock is either recovered from the data itself or sent as an external signal. The data is regular and bits synchronize with the clock signal. No start and stop bits are used. This allows for a higher baud rate when operating synchronously because bit timings have a certain guarantee and more bits can be used for data instead of as headers.

Whereas a UART has an internal clock signal and data on the bus can have somewhat sloppier and aregular timing. UARTs require start and stop bits and Asynchronous data is only synchronized with the start and stop bits.