The combine is used to harvest Wheat from a farm. It outputs seeds at the top and Wheat from any other side. It needs to be powered by a BuildCraft Engine to function. It is a good idea to attach pipes from the top leading back to the farm unit, thus supplying it with seeds.
The combine will harvest any fully grown wheat plant within a 21x10x21 area centered on the combine, collecting the dropped wheat and seeds. In addition to working with buildcraft pipes, it will also place harvested items directly in adjacent chests.