A turnpike is a busy road that charges users a fee to drive on it. If you drive through a toll booth, you know you're on a turnpike.
You can also call a turnpike a toll road because drivers have to pay a toll, usually when they exit, but sometimes also when they first enter the turnpike. This kind of pay-to-use road existed even before cars were invented. Today's turnpikes typically have toll plazas or booths, or special lanes equipped to collect tolls electronically. Highways that are free to use are called freeways or expressways.