A van der Waals gas is a type of gas that behaves differently from an ideal gas, especially at high pressures and low temperatures. In an ideal gas, the gas particles do not have any volume and do not interact with each other, but in a van der Waals gas, the gas particles have some volume and interact with each other through weak attractive and repulsive forces called van der Waals forces. These interactions can cause the gas to deviate from ideal behavior, resulting in changes in pressure, volume, and temperature that are not predicted by the ideal gas law.