What is a Powerline Network? How is it different from WiFi? And is Apple’s AirPort Express a Powerline Network? If so, how does it work?
I'd like to add to Jan's explanation, but I can't - it's perfect!
WiFi is a wireless network. It either allows a point-to-point (Ad-Hoc) connection between two devices or a infrastructure with a wireless access point/router and clients connecting to it. In most cases you can only setup a single network which all clients connect to. The maximum achieved bandwidth/throughput is primarily limited by the access point/router and shared by all clients.
Since signals go over the air there is always a certain security risk regardless of the encryption used.
PowerLine Network is a wired network technology utilizing (the existing) power lines in your house rather than network cables. You can also setup individual networks by using a different security key e.g. 2 pairs of adapters having separate traffic.
The maximum achieved bandwidth/throughput depends on the adapters used and is shared between all adapters on the same network. If you haven multiple networks running over the same power line they will "compete" for the bandwidth as the frequencies used overlap.
PowerLine is a rather secure network since one requires physical access to "pair" an adapter into the network in order to receive the data.
AirPort Express is simply the name of Apples standard wireless router. It has nothing to do with PowerLine network.