This is a basic breakout board for the FTDI FT232RL USB to serial IC. The pinout of this board matches the FTDI cable to work with official Arduino and cloned 5V Arduino boards.
It can also be used for general 5V serial applications. It allows a user to communicate with a serial UART through a common USB port.
This board brings out the DTR pin to allow an Arduino target to auto-reset when a new Sketch is downloaded. The 5V output is taken directly from USB and can power your Arduino.
This board has TX and RX LEDs that allow you to actually see serial traffic on the LEDs to verify if the board is working. This board requires a mini-USB cable.
Windows automatically installs the correct driver when you connect the adapter.
More information is available on the website of Sparkfun.