Energy is needed to operate machines as well as extracting items out of chests, automatic crafting tables and furnaces. BuildCraft uses the MJ power up till 1.6.4 and in 1.11.2 and 1.12.2, and the RF power in 1.7.2 and 1.8.
Engines are used to produce energy.
There are three types of engines:
Redstone Engine: Cheapest engine that produces the least energy. It only needs a redstone signal to work, but cannot be connected to a power grid and isn't able to run most machines.
Steam Engine (Stirling Engine): Produces a good amount of energy but needs burnable items to run.
Combustion Engine: Produces a lot of energy, explodes if not cooled with water. Needs fuel to run most efficient but also accepts oil (only accepts lava prior to 4.1).power plant.
Items can be extracted out of chests, furnaces and automatic crafting tables by attaching wooden pipes to them.