How do powerline network adapters work?

[MaxP779]

Just plug one in upstairs, plug another in downstairs and wham, simple as that?

What prevents a neighbor from hooking up his own adapter and using my network? Im not really up on wiring or how it works in houses so apologies if its a stupid question

 

[MaxP779]

I see, excellent. Ill be in a semi detached house so hopefully there wont be any problems there.