How do I understand the Stock Exchange Board?

I can't understand the stock exchange board. When I see the S&P or the DOW I am at a total loss on how to interpret it.
I don't understand the numbers on the board or what they mean. I don't know what they mean when they say " the stock exchange dropped 400 points today... or I don't know how to interpret the numbers and what is up or what is down.

Public Comments

1. The S&P and the Dow are just statistical summaries of stock prices that are supposedly representative of the whole stock market in some way.
http://en.wikipedia.org/wiki/S%26P_500
http://en.wikipedia.org/wiki/Dow_Jones_Industrial_Average

The numbers are stock prices measured in US dollars and cents. And they go up and down because the stock market is a kind of computerized auction where buyers are free to offer any prices they want and sellers are free to ask for any prices they want. A sale happens only when the bidding and asking prices coincide. And the price at which shares of a stock are sold becomes the latest price for all shares of that stock according to stock market rules.

2. The S&P 500 is a value weighted index published since 1957 of the prices of 500 large cap common stocks actively traded in the United States. The stocks included in the S&P 500 are those of large publicly held companies that trade on either of the two largest American stock markets, the New York Stock Exchange and NASDAQ. The Dow Jones average is a price weighted index computed from the stock prices of 30 of the largest and most widely held public companies in the United States.