In Grand Theft Auto V you can buy and sell stocks and shares on the stock market, which is accessed via your in-game web browser.
There are two types of stocks in Grand Theft Auto V: BAWSAQ and LCN. BAWSAQ shares are influenced by the Grand Theft Auto V community at large, so as players, for example, purchase more weapons and armor, shares in Ammu-Nation increase. The LCN shares by contrast are offline and only change based on your actions within the game.