A tanker is a sealed, tank-shaped trailer for moving liquids and other flowable bulk freight — gasoline, oil, chemicals, milk, water, and the like. It’s loaded and unloaded by pumping rather than by hand.
Tanker work takes special skill and, in the US, a tanker endorsement, partly because liquid sloshing inside the tank shifts the truck’s balance and changes how it handles. Hauling fuels or chemicals adds hazmat requirements on top. It’s the liquid counterpart to a dry hopper body.