What is "blood doping," and why does it matter?

Any time someone breaks a record, people inevitably bring up the possibility that they had some illegal enhancements. Just yesterday, gold medalist Ye Shiwen was accused of "blood doping" to improve her performance. You may be familiar with steroids and other drugs — but what on Earth is "blood doping?" And why would… » 8/01/12 7:00am 8/01/12 7:00am