Inside the big USB plug is a USB<->Serial conversion chip and at the end of the 36" cable are four wire - red power, black ground, white RX into USB port, and green TX out of the USB port. The power pin provides the 500mA direct from the USB port and the RX/TX pins are 3.3V level for interfacing with the most common 3.3V logic level chipsets.
Because of the separated pin plugs, this cable is ideal for powering and connecting up to the debug/login console on the Raspberry Pi. Connect the pins as shown to power the Pi and establish the RX/TX link. Then install the Windows XP/Vista/7 (Windows 8 not supported) or MacOS X PL2303HX.D drivers on your computer and connect with a terminal program at 115200 baud.
Also handy for hacking WiFi routers to install alternate OS's, or nearly any other TTL port. This is easier to use than an FTDI cable in many cases because the wires are separated.
*Please note: This cable is not good for Arduino re-programming such as a Boarduino, MENTA, Monochron, etc. because it does not have the DTR/RTS wire necessary for initiating the bootloader reboot sequence. For that we suggest an FTDI cable or FTDI friend.