The Watt rating is a measure of the real power only. The VA rating includes both real and imaginary (reactive) power.

For a purely resistive load, the reactive power is zero. In the case of a partially reactive (i.e. capacitive or inductive) load, however, extra current and voltage are "stored" during half the cycle, then "returned" during the other half of the cycle. Hence, reactive power is not useful for doing any real work. However, it nevertheless increases the total current flowing through the system's conductors and components. Therefore, it's important to specify the system's VA capabilities as well as its real power (Watts) capabilities. The difference between the two corresponds to the power factor (PF) of the system.

By the way, reactive power is usually measured in units of volt-amps reactive (VARs).