[BIGBOY]

QE is basically when the Federal Reserve buys a massive amount of bonds from banks and the gov't, which pumps newly created money into the financial system, and as a result of the liquidity is suppose to drive down interest rates and increase lending by banks. The money doesn't always go to lending, but also to bank reserves and to investment. The money has to be invested somewhere and the stock market has been where most of it has been invested since the crisis.

The short answer is QE puts more money into the system, which leads to more money in the stock market, which leads to higher stock prices, therefore rallying the market.