NASDAQ is a stock exchange, just like the New York Stock Exchange. The S&P 500 is an index, like the Dow Jones Industrial Average. GOOG remains on the NASDAQ exchange, but will soon be added to the S&P index of their "top 500" stocks.
Being added to the index helps investors. There are a number of mutual funds which attempt to "track" major indices like the S&P 500, so those funds will have to acquire GOOG shares in order to continue to track the index. There are other funds that are governed by rules trying to limit investments in risky stocks...limiting investments to S&P 500 stocks is one way to try to do this. So GOOG will be an option for many of these funds now.
