Pipe plugs are items. added in Buildcraft 3.5.2. Their purpose is to prevent connection between pipes, chest or machines which normally connect to each other. They are used in tightly packed machinery to avoid space wasting and the usage of different types of pipes (eg cobblestone and stone transport pipes)
The pipe plugs work on any kind of transport, conductive or waterproof pipes. Similar to the pipe wires, there is no way of getting the plugs back, without destroying the pipe they are put on. For full insulation of a pipe, you will need 4 pipe plugs.