You walked into the store and up to the Coca-Cola display where you usually purchase your cases of pop. To your astonishment, only one case is left on the display and as you are about to grab the case of soda, another man walks up and also wants to purchase it. Well, the store owner sees the two of you deciding who will go home with the pop and decides that he can make some money on this and says that he will sell it to whoever offers him more money for it. The store owner then decides to sell you the case of Coca-Cola because you offered him more money, thus driving the price of pop up for that case. This example is a lot like the stock market.
Stocks go down for the opposite reasons.
When more people want to sell stocks, the prices drop. This would be like if that store owner received too many cases of pop on accident and had to sell them. He would have a sale and offer them cheaper to attract shoppers. People who are trying to sell their stock offer lower prices to try to attract investors interested in buying the stock. The reasons that cause investors to want to buy and sell a certain stock varies.