A market order is a buy or a sell order that will execute at whatever price is being offered by the market at the moment the trade is executed. For example, if you're trying to buy $200 worth of bitcoin, and the current lowest ask is $8,500, then your trade will close at $8,500.

On some exchanges, such as Binance, market orders incur higher fees because they want to encourage people to set the price they're willing to pay, rather than taking whatever they can get. They call people who do market orders "market takers" and those who do limit orders "market makers."

A market order can cause you to get a price that is highly unfavourable. It'll take whatever it can get. If a lot of people are buying at the same moment you're buying, you may get a price that is way higher than the currently advertised price, because that's all it could find. A limit order will wait for the price to be triggered (although it may never trigger).

A market order is most useful when you want to get out of a trade right now and don't want to risk the price going further against you. They're often used for stop losses, since those are triggered automatically when you're not monitoring, and generally at levels where you think the price will continue to go against you.