Redstone

Redstone is the basis of power, both for simple things, like Pistons, and more complicated things.

Power
Redstone, like any good power source, has its unit. Redstone power is measured in red units, or RU.

Generation
There are multiple ways of generating RU. Redstone torches, Redstone blocks, active levers, daylight sensors, detector rails, pressure plates, and active tripwire all give a continuous signal (until deactivated or moved). Buttons give a short signal. Activated comparators act like the rest, but their signal strength varies. All sources, besides the comparator, emit 32 RU. These are all rudimentary ways, of course.

New Generators
Redstone Generator

Transmission
RU can be transmitted either by dust or by redsteel cables. Redstone dust loses 1 RU per square and can carry up to 16, while redsteel cables lose only 0.75 per square and can carry up to 32. More info about Redsteel Cables in their article.

Usage
All default things powered by redstone need at least 1 RU of power to function. Low energies are easy to acquire, being able to be generated easily with a simple lever or button. Some more complicated machines require much higher energies, only achievable with special machines.

New Machines
Redstone Furnace