What is difference between volt ampere and watt rating?

Watt rating is the amount of real power that is absorbed (or supplied). VA is normally the units given to the total complex power which is made up of the real power and reactive power. The formula for complex power is as follows.

S = P + jQ

P is the real power which is used by the resistive elements of the component while the reactive power (imaginary power) is that power which the capacitive and inductive parts of the circuit cause to flow.

Watt ratings are always lower (or equal) to the VA rating due to the fact that it neglects imaginary power that is flowing, because of this both ratings should be taken into account.