BuildCraft is one of the major mods in the Tekkit Classic Modpack. It adds a variety of automated mining, building, crafting and engines as well as pipe transport and sorting to the Minecraft experience.
The BuildCraft wiki can be found here.
In order to play Buildcraft properly on a server, the server owner must op the mod itself, by typing '/Op [Buildcraft]'. This is because some elements of Buildcraft, such as the Quarry, need permission to interact with the environment.