An invisible fence is a buried wire that is linked to a unit that is plugged in to an outlet (like in your garage). Your dog wears a collar that is programmed to either beep or shock them when they cross the barrier. I think some new systems also spray something that smells bad to the dog. They also come with warning flags that you put over the wire so the dog can get used to where the line is without suprising the crap out of them. I personally wouldn't use one that would shock my dog, but I hear all systems work pretty well. I think you can find the systems in most stores.