An Optical Polarizer is an optical element that screens light and split it into two beams with a specific ratio according to its polarization state. This article unfolds a basic concept and working principles of different categories of birefringent crystal polarizers including Glan polarizers (Glan Thompson, Glan Taylor), Wollaston Polarizers, and Rochon Polarizers. In the text, we also introduced two kinds of optical parts related to polarization: Polarization Rotator and Depolarizer.