VA (volt-ampere) is the unit for apparent power. W(watt) is the unit for real power. The relation between the two is:
[tex]real power = power factor \cdot apparent power[/tex]
The load that you may connect to a transformer could have a reactance which results in a power factor less then 1 for the load and so the total power consumed by the load would be it's apparent power (expressed in VA) and not it's real power (expressed in W). That's why the power rating of a transformer is given in VA and not in W.