WE have an invisible fence system - I bought mine from Canada from
this guy as it worked out best value for money.
Essentially you trace a wire around the area you want to restrict the dog to - can be any length - mine runs to around 800 ft. The ends of the wire connect to a low voltage transmitter, and then the dog wears a collar with a receiver. If he gets near to the fence he gets an audible bleep. If he starts getting closer still the beep gets faster and if he gets too close he gets a static shock. Training was straightforward - one shock was enough to deter him and now he knows the area without even waiting for the bleeps!
It's a good system because it means we can leave our dog out without fear of him straying or causing a nuisance.