BuildCraft is a modular modification for Minecraft originally created by SpaceToad and now maintained by Sengir and Krapht. BuildCraft introduces ways to transport items and liquids, as well as generate and transport power. This is in addition to it's ability to help automate crafting and construction.
BuildCraft under 1.2.5 was modular, which meant that the the player could choose which submods s/he wanted to install. BuildCraft test releases for 1.3.2 and 1.4.2, and the final releases for 1.4.5 came as one mod file, as have releases since then.
As of 3.4.2, BuildCraft now has it's own Creative Tab for indexing its items.