The Iron Pipe has up to five inputs and one output. It will send all incoming items in one direction. The direction can be changed by right-clicking the pipe with a Wrench. The output direction can also be changed by Redstone current. Every time it either loses or gains Redstone power, it will cycle to a different output. However, it will never output to a Wooden Pipe. This combination of properties allows for players to accurately control its output direction from afar, via Redstone wiring. The Iron Pipe will pop out blocks if they try to go through the barrier that makes them go the correct direction as of BuildCraft 2.0.
Iron Pipes will occasionally bug when they have more than 2 additional Pipes connected. This bug presents itself as the pipe visually showing it will allow items to pass through in a direction, but instead the items either bounce back the way they came or move off in one of the blocked directions.
Iron pipes can also be used as an extremely handy alternative to Obsidian Pipes for dealing with overflow. Place two iron pipes in a row, both pointing the same direction, and put the second one so it flows directly into a machine, such as a furnace. This way, when the furnace has, say, copper ore inside, but another ore comes through the pipe, it won't get "popped" out of the pipe and sit there to be picked up by you or despawn. Instead, it will turn around in the second iron pipe and head backwards into the first iron pipe... which will then send it forward again, repeating over and over until the furnace has finished with the copper ore and allows a new ore inside. A limit to how many blocks can be held in suspension like this has not yet been found.