The simplest of the order types, a market order is a buy or sell order to be executed immediately at the current market prices. As long as there are sellers and buyers, the market order is filled. Marker orders are used when certainty of execution is a priority over the price of the execution. Essentially, they are transactions meant to be executed as quickly as possible at the existing market price.
We don't know everything about the markets. We're just devoted to learners. Taken from those smarter than ourselves, here's how we define Market Order.