Most Machines are blocks that use or create power/fuel to perform an action more efficiently than a player, or to perform an action that a player can not perform manually. Not all machines use fuel or energy and some need other machines to be built before they can be used.
There are four main types of machine in Industrial Craft;
Most Machines in Buildcraft require Gears to build.
Buildcraft Machines are either Engines or Machines that require engines to power them while they process or collect materials.