Minecraft is a 'sandbox' game, which means it has no story, and allows total creative freedom. When you start a new game, a world is generated, filled with artificial plant life and animals. This map is about 7 times the size of the planet earth. There are oceans, deserts, forests, jungles and plains. Depending on which 'game mode' you choose, you can either have god-like powers - the ability to fly and create without limits - or try and survive the harsh world, building contraptions and buildings to keep you safe from the monsters at night. What you do in either of these modes is up to you.
Here are some examples of how Minecraft has been used to create
'Redstone' is Minecraft's way of introducing circuits and logic gates into the sandbox world. It can be powered by 'Redstone torches' and be used to light lamps, push pistons, and power rails. From it, many complicated constructions can be built.
A 4-bit computer
A fully working display
The Internet
A 24-Hour Clock
You can read more about Minecraft in education here