Forecasters were expecting about 40-60 "shooting stars" per hour last night. But what causes these fantastic displays of shooting stars?
Meteor showers occur when the Earth passes though the debris left from a passing comet. As a comet streaks by, it leaves behind a trail of dust and tiny debris as the sun burns off part of it's exterior (kind of like how a slug leaves a trail of goo).
In the Perseid's case, it's leftovers from the comet Swift-Tuttle, which last passed by in 1992. As the Earth passes into the debris field, the dust and debris hits the atmosphere and burns up -- what you see as a shooting star.