If the US defaulted, financial markets would lose faith in the United States, the dollar would weaken, and stocks would fall. The U.S. credit rating would almost certainly be downgraded, and interest rates would rise broadly on US debt offerings, every holder of the trillions of dollars in US debt would see there investment go down in value. This would result in interest increases on consumer loans, making products like auto loans and mortgages more expensive for families who are subject to interest rate changes or taking out new loans. These and other consequences could trigger a recession and a credit market freeze that could hurt the ability of American companies to operate. The ramification would effect businesses and governments around the world.
Everyone seems to assume this can not happen but in fact, it certain can happen as politicians play a deadly game of chicken betting that the treasury has more funds than they claim or democrats will give in. And more far right republican who are willing to blowup the economy rather than back down makes it more likely. It probably won't happen this time but I think it will eventually as the right and left move further and further apart.