The stock market is a market on which shares of public held companies are sold. The companies on the market are traded by brokers or investors and this depends on the value of each share of stock. The buyers know how much they are willing to pay and make an offer called a bid. The sellers also know how much they want to sell for and list their price called the 'ask'. This will either make the price of the stock to go up or down in value. For more details, visit http://money.howstuffworks.com/personal-finance/financial-planning/stocks.htm.