UART features

Usually, the UART logic block offers:
An interrupt for tx-byte and rx-byte
Two FIFOs, for input and output (8, 16, 64 byte)
FIFO-related interrupts, with configurable thresholds

To run a UART device, you need to provide a clock signal
Usually, the clock frequency is 16 times the bit rate
If your UART is in-uC, you usually have a clock divider
External UART devices feature a divisor and associated registers
The clock doesn't need to be very sharp (4% error is acceptable)

The standard shows its age: DTE and DCE are different
To connect two DTE devices you need to swap wires

Usually there is RTS/CTS handshake (hardware flow control)
And sometimes there is XON/XOFF handlshake (software flow control)