Stock trading is buying and selling shares of publicly traded companies.In the stock market, for every buyer, there is a seller. When you buy 100 shares of stock, someone is selling 100 shares to you. Similarly, when you go to sell your shares of stock, someone has to buy them. If there are more buyers than sellers (demand), then the stock price will go up. Conversely, if there are more sellers than buyers (too much supply), the price will fall.