The van der Waals equation is an equation of state that describes the behavior of real gases. It modifies the ideal gas law by accounting for the volume occupied by gas molecules and the attractive forces between them. This makes it more accurate for predicting the properties of gases under various conditions, especially at high pressures and low temperatures. The equation is expressed as (P + a(n/V)^2)(V - nb) = nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and a and b are constants specific to each gas. These constants represent the strength of intermolecular attractions and the volume occupied by gas molecules, respectively.