The plwk400 plugs into your electric outlets and uses the electrical wiring to carry the signal to the area where you want to extend your wifi to. You then plug the 2nd device in and it acts as a wireless router.
The "PL" stands for "Power Line", meaning that thia device "captures" the network from a signal that was mixed into the electrical circuit, and enables it wirelessly to your devices... I believe the "N Extender" does not work with the "PL" function.
The RE1000 extends the wireless range. The PLWK400 extends a "hard wired" 4 port hub to a convenient electrical outlet, perhaps near a home theater setup. Hard wired ethernet is the preferred way to stream digital content. Depending on your network wireless would not be expected to do as good a job.