When you buy and sell shares through your broker, you will be given the option of what type of order you are placing.

Limit order - A limit order is the order price you are willing to pay up to for a buy order or sell down to for a sell order. It is the limit of what you are willing to accept, hence the name.

An example of this is that you place a sell order for a stock at $28.00. By setting this, you are not willing to accept a price lower than $28.00. A limit order can be placed at any time when the market is open as well as out of market hours.
At Market - When you place a market order, you are accepting the best market price on offer. The order has no specific price when the order is placed. The sell order takes the priority buy price currently in the market, while a buy order takes the priority sell price currently in the market.