Can a Brit explain football to this American?

I never watch soccer. OK football, I never watch football but I DID watch the USA vs Slovenia match and it actually seemed pretty exciting but at the end when the US scored the final goal, the goal was taken away. I don't understand the rules of soccer..football. What happened? Usually in American football and basketball and hockey, the referee says player number whatever committed whatever foul. What kept that final goal from being counted?