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 3.3V Arduino boards. It can also be used for general serial applications. The major difference with this board is that it brings out the DTR pin as opposed to the RTS pin of the FTDI cable. The DTR pin allows an Arduino target to auto-reset when a new Sketch is downloaded. This is a really nice feature to have and allows a sketch to be downloaded without having to hit the reset button. This board will auto-reset any Arduino board that has the reset pin brought out to a 6-pin connector.
The pins labeled BLK and GRN correspond to the colored wires on the FTDI cable. The black wire on the FTDI cable is GND; green is CTS. Use these BLK and GRN pins to align the FTDI basic board with your Arduino target.
This board has TX and RX LEDs that make it a bit better to use over the FTDI cable. You can actually see serial traffic on the LEDs to verify that the board is working.
This board was designed to decrease the cost of Arduino development and increase ease of use. Arduino Pro and LilyPad boards use this type of connector.
If you'll be doing a lot of programming and would find it a nuisance having to plug in and unplug this adapter to gain access to your board's serial lines each time, consider the new FTDI SmartBasic board which has a multiplexer that allows pass-through of the serial lines. Just note that the SmartBasic is set up for 5V usage by default; you'll have to change a solder jumper to set it for 3.3V signals.
FT232RL USB-to-Serial Board Resources