What is Common Mode Chokes

A common mode choke is an electromagnetic component that blocks high frequencies by passing direct currents (DC) and alternating currents (AC) through an electrical circuit.
What is the difference between a common mode choke and transformer? A common-mode choke is a sort of hybrid where the inductance is used to suppress common-mode signals, but the transformer action allow normal-mode signals to readily pass. The difference is basically in usage. A choke uses the inductance as a primary characteristic to affect a signal. In a transformer the inductance is secondary and is just to establish the magnetizing current with the transformer’s primary purpose being to transform one signal level (or impedance) to another.
The choke normally consists of insulated wire that is wound around a magnetic core. It is an inductor that is utilized to block high-frequency, alternating current (AC) in a circuit while allowing lower frequencies, or direct current (DC), through.

A common-mode choke has 4 pins. A common-mode choke is typically drawn as two inductors separated by two lines.

The two lines represent the inductive coupling between the two inductors.