Market order vs limit order

A market order buys or sells immediately at the best price currently available. It guarantees execution but not price, so in a thin or fast-moving market you may pay more, or receive less, than the last quoted price.

A limit order sets the worst price you are willing to accept: a maximum for a buy or a minimum for a sell. It guarantees price but not execution. If the market never reaches your level, the order simply waits in the order book until it fills or you cancel it.

Many exchanges charge lower "maker" fees for limit orders that add liquidity to the book and higher "taker" fees for orders that fill immediately. A stop-loss order is a related tool that triggers a sale once the price falls to a set level. During sharp crashes it can fill well below that level.