The RedPower mod for Minecraft is a mod that greatly improves and expands the mechanics of redstone. It also adds machines and integrated circuits that allow the player to make complex systems to manufacture items or perform specific tasks. It is used a great deal for factories, as its automatic crafting tables and electronics-based piping systems allow items to be transported and crafting automatically. There are a large range of machines available, from simple transposers that suck items out of chests or pick items off the floor when powered with a redstone signal, to deployers that emulate a right-click action (and therefore can be used for tasks such as filling buckets or lighting fires). Pipes (called pneumatic tubes) are also provided for item transportation. The real strength of RedPower however, is its expansion and improvement on the redstone system in Minecraft. Vanilla redstone is very block and placement-specific, has a signal that must be boosted with a repeater every 15 blocks, and has a very limited number of components. Therefore, many people find it too tedious or irritating to build electrical systems using redstone. This is where RedPower becomes very useful. It replaces redstone with its own wire called red alloy wire, which works like redstone except it can be put on any face of a block. It provides many integrated circuits, which condense a large and complicated vanilla redstone circuit into a single block. For example, a toggle latch is made in vanilla with a 5x5 extremely complex redstone circuit. It is one block in RedPower. This functionality is fantastic for making complex systems and circuitry, making it the leading mod for creating automated systems and factories.
The link to the RedPower wiki (from tekkit classic) can be found here.