In case you haven't been keeping up with the news lately, the United States government just lost its AAA credit rating from Standard and Poor's late on Friday. The lowering of the credit rate to AA+ is the first ever for the government in history, even though Standard and Poor's did admit to a $2 trillion accounting mistake. The decision is basically just one company's opinion (two other credit agencies, Moody's and Fitch, still rate the US as AAA at least for now), the financial aspects will likely affect the economy for years to come.
Credit ratings are not just given to countries but also to corporations. As Business Insider reports the US used to have a whopping 60 companies that had a AAA rating. In 1995 that number went down to about 30 or so. Today, just four US corporation have a AAA credit rating and one of them happens to be Microsoft. Yes, the company that Bill Gates built, now has a higher credit rating, at least according to Standard and Poor's, then the US government (Exxon Mobile, Johnson and Johnson and Automatic Data Processing are the other three AAA US-based companies).
Basically, having an AAA credit rating from an agency means that if you have a bill that is supposed to be paid by a government or company you have the best chance of getting your bill paid back in full. The US government's lowering of its credit rating to AA+ is still a fairly safe bet but not as safe as having the AAA rating. Indeed, the US Treasury Department is already disputing Standard and Poor's decision, calling it "flawed."