The 16550A is the most common
UART used for PC serial ports today. They differ from the original
IBM PC 8250 UARTs in that they have a 16 byte FIFO buffer. The old 8250 UART generates an interrupt for every received byte, and that
interrupt must be serviced
immediately or else the byte is lost when the next one comes in. The 16 byte FIFO allows
software a bit more leeway in responding to
serial port interrupts.
This was a common issue in the early days of DOS gaming; games of modem to modem DOOM were greatly improved when the UART was upgraded from 8250 to 16550A.
One should also note that the shortlived 16550 (with no A) was a buggy version of the chip with a non-functioning buffer. There was also a 16450 UART that also lacked the buffer. Classic 16550A's are obsolete today; they don't support speeds over 115.2Kbps.