The major feature of BuildCraft is the pipes. Pipes are used to transport items, fluids, and mechanical energy between inventories/machines. Pipes can be combined with Pipe Sealant to allow them to make Fluid Pipes that can transport liquids or with Redstone to make Kinesis Pipes to allow them to transport mechanical energy.
In BuildCraft, there are a variety of machines that can be used to automate processes. They all require BuildCraft power to function, provided by one or more engines.