You'll need two adapters to get everything started. Plug one into an electrical socket and then plug your router into that adapter. Then plug another adapter into a socket near your television or whatever device you want to get online. That adapter can hold up to four Ethernet cables and will use a secure signal pushed through the electrical system to take all four devices online.
It's a clever idea and could be very useful to those who have Internet-capable devices all around the house, but don't want to string cables throughout their home or rely on a fading wireless signal. The adapters can be purchased for $59.99 each.