The stock market is a complex system where shares of publicly-traded companies are issued, bought and sold. There are many factors that determine whether stock prices rise or fall. These include the media, the opinions of well-known investors, natural disasters, political and social unrest, risk, supply and demand, and the lack of or abundance of suitable alternatives. The actual price of a stock is determined by market activity.. The best time to buy is when others are pessimistic. The best time to sell is when others are actively optimistic