A buy order is an instruction to a broker or trading venue to purchase a stated quantity of a security. It is one of the two sides of every order, the other being a sell order. A buy order states the instrument, the quantity, the type of order and how long it stays valid.
The type decides how the order is priced. A buy market order is filled at once against the lowest offers available, so the buyer is sure of the trade but not of the price. A buy limit order names the highest price the buyer will pay and waits in the order book as a bid until a seller meets it. A buy stop order is triggered only when the price rises to a set level. A buy order may open a new long position or close an existing short one, in which case it is called buying to cover.