This breakout board pairs an SP3485 RS-485 transceiver with an FT232RL USB UART IC to convert a USB stream to RS-485. The SP3485 is a half-duplex transceiver, so it can only communicate one way at a time, but it can reach transmission speeds of up to 10Mbps.
The TXDEN pin of the FT232RL is connected to the transmit and receive enable inputs of the SP3485, this line is used to control the transmission mode of the RS-485 transceiver. The FT232RL will enumerate as a virtual COM port. Drivers are available for Windows, Mac and Linux. With Windows the driver will install automatically.
More information is available on the website of Sparkfun.