Redstone

One major aspect of minecraft is the redstone. People that redstone are coined by the community as "Redstoners" and can create something as simple as a far or something as complex as another game. Redstone is most similar to electricity to us, but a lot simpler. It can be run through gates and comparators to either completely negate the power or just reduce it. In this way you can get a simpler version of binary and create some simple things. If a signal is on, then you can activate a functional block on the other end. 

The basic blocks of redstone include the the torch, repeater, comparator, redstone block, and the piston. A torch is a source of power and can be deactivated by powering the block it is attached to with strong power. After redstone extends 15 blocks past its source the power stops, the power must be renewed at this point with a repeater which delays the energy from going through by 1-4 ticks depending on its setting. Multiple repeaters can be used to make a clock which outputs redstone signal at regular intervals. Comparators compare the signal strength of two different lines of redstone, the main one is either decreased by the power of the secondary or just shut off depending on the settings of the block. A redstone block is another source of power that cannot be shut off, but can be shoved by pistons. Pistons come in one of two varieties, sticky and normal and push up to 15 blocks in one direction with the exception of a few blocks. The sticky pistons can also retract block while the normal one only push, these can be used to create hidden doors that can be hidden within walls or the floor.

Gates and structures of redstone can be a bit complicated to explain, so I have made a video that can help you with some useful tutorials even for experienced players. All of this is done in creative mode, but normally you would find redstone deep beneath the earth.I might even leave a link to one of the most impressive thing of all time. Most of the crafting recipes can be found on the wiki which I will leave a link to.

Link to redstone page: http://www.minecraft.gamepedia.com/Redstone

My video: http://www.youtube.com/watch?v=vG5WmZ9t1LQ