There are two order execution options: Market or Limit.

• Market orders are meant to execute as quickly as possible at the current market price (highest Bid for Sell orders and highest Ask for Buy orders).
• Limit orders set a maximum or minimum price at which you are willing to buy or sell.

Some exchanges don’t allow Market orders, only Limit orders, such as Bittrex and Poloniex.  That’s to protect you from executing at a bad price, which is a risk particularly for Assets with low liquidity.