The Gulf of Mexico Deepwater Horizon Oil Spill: Some Background and What It Means

"We have all been reading about the blowout that led to huge fire and sinking of the Deepwater Horizon drilling rig in the Gulf of Mexico. Now there is news that there is a huge oil spill coming from the underground pipe where this occurred, and there is a possibility that it will be months before it can be stopped. What does this all mean? How could this happen?"