The term "market order" refers to a buy or sell order that is to be executed instantly at the current market price. Market orders are filled when sellers and buyers are willing to exchange. Market orders are used when certainty of execution is more important than the price of execution. You can either choose the Amount to buy or sell assets/funds/cryptos when placing a market order.
Buy and Sell can be used to place orders when the market price fluctuates drastically, in that case, the order might get filled at an unexpected price.