How do these powerline networking adapters work?
If you want a simplistic answer. Here it is
You have the power source. It gives you 220 Volts at rather low frequency (as noted: 50-60 Hz). You connect a capacitor with a low value and an inductance of a low value to this line this way: low inductance doesn't allow 50-60 Hz to pass in - it shorts such currents to the ground (being placed after capacitor! not to short everything at all), on the other hand capacitor in it's place again doesn't allow low frequency in and passes through high frequency, which we send or receive. The other party does the same - we have a working transfer line.
But, ofcourse we connect different devices to our power socket. What happens in such a situation is simple too: mostly they are - ac/dc convertors, which use either transformer, or impulse scheme. This schematic doesn't allow high frequencies, eg they provide huge resistance to it. But we remember the capacitor in both our devices - it provides low resistance, so our-generated high frequency signal takes the easiest way: our receiver.
Ofcource there will be noise and leakage, our coding system and filters will deal with it.
That's the picture in it's simplest form. Hope this is what you were asking ;)