The assembled ribbon cable consists of two 2 * 20 Pin Socket connectors assembled on a 150mm length of 40 Pin Ribbon cable.
The Raspberry Pi is equipped with a row pin headers. These aren't just for show however, these GPIO (General Purpose Input/Output) pins can be used to access a whole new world of fun with your Raspberry Pi. Through programming the Pi, these GPIO ports are able to undertake a variety of tasks including playing with LEDs, running motors and sending and recieving data.
So, if you want to get your Raspberry Pi interating with physcial hardware electronic systems such as a breadboard, you'll most likely want to bridge the connections with a ribbon cable and socket connector.
Here's a demonstration of what you can do with the GPIO pins: