The document discusses different types of transformers, including their basic principle and construction. There are two main types - core type transformers, which have a core with one window that winding encircles, and shell type transformers, which have a core that encircles sandwich-type windings within two windows. Transformers work by inducing voltage in a secondary winding through electromagnetic induction from a varying magnetic flux produced by an alternating current in a primary winding. They are used to change voltage levels for power transmission and distribution as well as for measurement applications.