After a company goes public (IPO) and its shares start trading on a stock exchange, its share price is determined by supply and demand for its shares in the market. The mechanism used to set prices is facilitated by exchange-designated “market makers”. “Market makers” continually post “bid” and “ask” prices to facilitate a tradable market in the stocks in which they are designated to make markets. As buys and sells come in, the market makers move their quoted prices up or down in response. These are the scrolling prices you often see on news channels.