The constants a and b have positive values and are specific to each gas. The term involving the constant a corrects for intermolecular attraction. Attractive forces between molecules decrease the pressure of a real gas, slowing the molecules and reducing collisions with the walls.
The higher the value of a, the greater the attraction between molecules and the more easily the gas will compress.
The b term represents the excluded volume of the gas or the volume occupied by the gas particles.
Notice that the van der Waals equation becomes the Ideal Gas Law as these two correction terms approach zero.
The van der Waals model offers a reasonable approximation for real gases at moderately high pressures. Additional models have been subsequently introduced to more accurately predict the behavior of non-ideal gases.
Source: Boundless. “Van der Waals Equation.” Boundless Chemistry. Boundless, 12 Nov. 2014. Retrieved 16 Dec. 2014 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/gases-5/deviation-of-gas-from-ideal-behavior-56/van-der-waals-equation-269-8307/