How does a microcontroller development board use USB cable to transmit serial data?
The serial interface is actually a virtual com port that communicates using the USB interface. Your microcontroller does not need to convert the serial input/output between normal logic levels and the higher RS-232 voltages...the logic signals from the UART go directly to the USB interface. Software on the host computer must be able to recognize the virtual serial port and provide the appropriate driver, so it looks just like an old fashioned serial port to the host software.