Nipples
Nipples are the fittings with having male threads in both sides and they are used for connecting two pipes to each other. The both sides of nipples can be not only in the same size, but also in different sizes.
Sockets / Coupling
Sockets are the fittings with having female threads in both sides.
Plug
Plugs can be either male or female, without a hole inside used for preventing the exit of fluid from the system.
Reducer
Reducers are the fittings whose outer sides are male while the inner sides are female and also one side is bigger than the other.
