According to Wikipedia a Sub-Prime is typically, customers are those who do not qualify for prime market rates because of blemished or limited credit. 

Business Week believes the sub-prime market is a tinder box ready to blow.  And rightfully so, even thought the real estate economy has cooled off, people are defaulting, some lenders are still pushing sub-prime loans to people who could easily default. 

Finally, some Wall Street lenders are waking up to the looming, impending problem.  Inman News posted how Wall Street investors are starting to become weary of sub-prime loans, saying they’ll only keep the money flowing if the wholesaler buys back a big chunk of dud loans.  But with wholesalers opting to file Chapter 11, it leaves a lot of doubt as to how to recap the money.

Here is the link to Business Week.