The market is like an ongoing auction. The 'price' is what the last buyer and seller agreed upon. When you send a broker into the market to buy for you, he has to join the auction and hope that someone is willing to sell at your buying price. If you put in a 'limit order', you set a price at which you're willing to buy. If no one will sell at that price, the broker comes home empty-handed. When you put in a market order, you're taking a chance on price, but you're sure to get your shares.