Members of the New York Stock Exchange are the brokers who actively trade stocks on the floor. When your stockbroker executes your order to sell, it is not completed until one of the brokers on the floor of the New York Stock Exchange finds another broker to buy it. That is why it might sell for a slightly different price than it was when you placed the sell order.
The NYSE is moving to an all-computerized trading system to compete with the NASDAQ, which is all-electronic.

