Actual repeaters are mostly garbage, as you put them near the edge of an existing signal (so it already has a weak signal to begin with), then it just broadcasts it's own network from there and funnels it all back through the initial network. Adds more layers to the wireless connection and in general just performs like shit.
The idea behind the powerline adapters is that yes, it basically makes cat5 out of your copper electrical in the wall. What this allows you to do is get "wired" internet to a far room of the house, buy another wireless router, put it in bridge mode (removes the routing function, so it works like a switch with wireless basically) then set it to broadcast an exact copy of your existing wireless network (same ssid, encryption, password etc). This way you see only one wireless network, but devices will connect to whichever wireless broadcast point has a better signal, depending on where you are in the house. I'm pretty sure you can buy "dumb" wireless broadcasters that have no routing functions by default that you can use for this, but they are almost more expensive that just buying a regular wireless router and are far less flexible and more annoying to configure.
Powerline adapter DO have limitations with how far they can push a connection through the existing copper. Most are like a 300-500 foot limit or there abouts? Maybe they've improved since I had to install one, who knows. They also can have trouble if you have old or generally shitty wiring in the house, but it's super easy to test if they are gonna work or not. Just plug both ends in, then wire a laptop to the far end. If it gets a wired connection, you're done, there's literally no other setup involved in the powerlines.
