This board converts a USB connection into 5 volt TX and RX that you can connect straight to the Arduino Ethernet or any other boards with an FTDI cable-compatible connector.
It features the Atmega8U2 programmed as a USB-to-serial converter, the same chip found on the Arduino Uno
. The USB Serial module has an ISCP interface, allowing you to reprogram the chip when placed in DFU mode
The pinouts on the connector are compatible with a standard FTDI header (as well as the Adafruit and Sparkfun USB-Serial adapters).
for more details.