A van is a roomy vehicle with a boxy shape that's useful for transporting extra people or extra stuff. If you're moving some furniture across town, your mom's van will come in handy.
There are many kinds of vans, from commercial vans that deliver appliances to airport vans that transport hotel guests and their luggage to the small vans parked in many suburban driveways. A less common meaning of van is "forefront," and this meaning is usually used when talking about military movement: "The general was at the van of the troops as they surged forward." The vehicle definition comes from caravan, "large covered carriage used for conveying passengers."