The FTDI cable is a USB to Serial (TTL level) converter which allows to program Arduinos that does not contain a USB port. The I/O pins of this FTDI cable are configured to operate at 5V.
The FTDI cable is designed around an FT232RQ, which is housed in a USB A connector. The other side of the cable is terminated with a 0.1" pitch, 6-pin connector with the following pinout: RTS, RX, TX, 5V, CTS, GND (RTS is the green cable and GND is black).
You can use this cable to program your Arduino Pro, Pro Mini and Lilypad and in USB to Serial UART communication.