Who the hell are Standard & Poors?

Anyone with a pension, any kind of property or savings will be hit by the sharply increased turmoil in the 'markets' this week. Sensible folk find it hard to get their heads round this. The business that has suddenly seen its share price dive is exactly the same business that last week was seeing a steady share price rise. It seems that this increased turmoil is largely caused by the opinions of a private company called Standard & Poors. As Nobel Prize winning economist and New York Times columnist Paul Krugman notes "It's hard to think of anyone less qualified to pass judgment than the agencies [S&P & others] who [highly] rated sub-prime securities". If anyone knows & can explain in simple lay terms why markets pay any attention to them please do so.