The USB 2 Serial Converter converts a USB connection into a 5V TX and RX that you can connect to boards with an FTDI cable-compatible connector (e.g. Arduino Ethernet).
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 ICSP 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).