How it works
When you place a market order, it is matched, in order, against the best-priced buy or sell orders currently available in the market, and in principle it executes right away. It is an order type that prioritizes getting the trade done over controlling the price.
Difference from limit orders, and things to watch for
A limit order, by contrast, specifies the price at which you want to buy or sell (see the Limit Order page for details). Market orders are more likely to be executed, but for thinly traded stocks there is a risk of execution at a less favorable price than expected.
For stocks subject to a daily price limit (limit-up or limit-down), even a market order will not execute at a price beyond that limit.