The system will work by a chip being inserted into the match ball. When the ball crosses the line it will set of a buzzer that the referee will hear, allowing the referee to know that it is a goal.
Finally action has been taken to reduce human error as it comes following several actions the most famous being Roy Carrol's blunder, where the goal was disallowed, despite the ball crossing the line. A similar event took place to Stevenage when they played Newcastle and Alan Shearer scored but the ball didn't cross the line following Mark Smith's overhead kick. the goal was allowed and Stevenage went on to lose the match 2-1.
Despite this technology being introduced FIFA will not launch any more technology and are quite happy for human error to take place, rather than install a third umpire, which in other sports has been very sucessful.