There are many kind of transformer in the market, they are very different. The have the similar structure which has two sets of coils wind the same iron core. The two sets of coils called primary coil and secondary coil. The current go into the primary coil and out of the secondary coil. If the primary coil is more than secondary coil, the voltage of secondary coil will decrease. This is step down transformer. If the primary coil is less than secondary coil, the voltage of secondary coil will raise. This is boosting transformer.
The principle of transformer is not complex. According to the electromagnetic induction principle, when a conductor put into a varying magnetic field and it can induct current at the same time. When we put transformer into the ac network, the current imported to the primary coil and around the current will create magnetic field. Due to the direction of alternating current is always changing and will create the magnetic field with synchronous change. The magnetic field walk along with iron core to form a closed loop. Because of the size and direction of magnetic field are changing all the time and induced current by secondary coil. With the same voltage in each coil, the more secondary coil, the higher voltage from secondary coil.