There's a lot more to trading stocks than just "buy" and "sell," and it's easy to be confused by all the types of orders you may have heard about. So here's a quick guided tour.
Market order: A customer order for immediate execution at the best price available when the order reaches the marketplace. The most common type of order, a market order is nearly always filled, since no price is specified.
Limit order: An order to execute a transaction only at a specified price (the limit) or better. A limit order to buy would be at the limit or lower, and a limit order to sell would be at the limit or higher. Limit orders are used by investors who have decided on the price at which they are willing to trade, and may be more expensive to execute than market orders.
GTC (Good Till Canceled): A variant of a limit order either to buy or to sell a security, this order remains in effect until it is canceled by the customer or executed by the broker.
Fill-or-Kill: An order that is sent to the floor for immediate execution. If it cannot be filled immediately, it is automatically canceled.
All or None (AON): A limit order either to buy or to sell a security in which the broker is directed to attempt to fill the entire amount of the order or none of it. An all-or-none order differs from a fill-or-kill order in that, with an all-or-none order, immediate execution is not required.
Day Order: An order that terminates automatically at the end of the business day if it has not been filled.
Stop Order: An order that trades after a specified price level has been reached. It may be time-limited as well, as with a day order or GTC order. A stop order guarantees execution, but not price, if the specified price level is reached or traded through.
Depending on how active a trader you are, you may use several of these types of orders. But for most investors, market and limit orders will satisfy your needs.