The van der Waals equation is based on a modification of the ideal gas law and approximates the behavior of real fluids, taking into account the nonzero size of molecules and the attraction between them. The equation uses the following state variables: the pressure of the fluid, total volume of the container containing the fluid, number of moles, and absolute temperature of the system.  The parameter b, is called the excluded volume per mole. This has an effect on the volume parameter because the volumes of the molecules in the gas are not point masses, and therefore there volume is excluded from the volume of the container.

The parameter a is called the proportionality constant and provides a measure of how strongly the molecules attract each other.  van der Waals reasoned that the intermolecular atrractive forces would have an influence on the pressure.  This equation is useful in that the relationship allows for solving for any of the state variables which may be under investigation.  

van der Waals knew that to fully understand the relationship between pressure, volume, and temperature you had to take into account the volumes of molecules and the intermolecular forces between them .  These forces are now generally known as "van der Waals forces".

Notes

This equation would be used to answer a problem along the lines of:

Calculate the temperature of 0.600 mol of Oxygen in a 0.1 L container under 27 atm of pressure.