Stock prices change whenever a trade executes at the bid or ask price, which happens constantly, according to The Online Investor. The last price indicates the most recent price at which a trade executed, but it may differ from a stock's bid and ask prices.
The bid is the highest price that a buyer is willing to pay, and the ask is the lowest price that a seller is willing to accept, explains The Online Investor. For example, a buyer may place a bid at $20 for shares of a particular stock, whereas a seller may ask for $20.25. A trade executes, and the stock price changes, when demand increases and a buyer is willing to pay the ask price, or when demand decreases and a seller is willing to accept the bid price. This can happen many times throughout a day.