The definition of a power transformer as stated in IEC 60076 Part 1 is as follows :-
A static piece of apparatus with two or more windings which, by electromagnetic induction, transforms a system of alternating voltage and current into another system of voltage and current usually of different values and at the same frequency for the purpose of transmitting electrical power.

There are two general types of transformer construction - fluid filled and dry type.

In fluid filled transformers , the cooling and insulating medium used within the transformer is transformer oil.
The dry type Cast Resin Transformer , cast resin insulation and air cooling is used.