Order types

Market, limit and stop.

The three you need

A market order executes immediately at the best available price. A limit order waits for the price you name and may never fill. A stop order becomes an order once a price is reached.

Market orders guarantee execution, not price. Limit orders guarantee price, not execution.
In a fast or thin market the difference between the two can be large.

Time in force

Orders can last for the day, until cancelled, or until a date. Leaving a stale order open for months is a common way to be surprised.