Cataclysmic variables are systems composed of two stars. One of the stars is similar to our Sun and the other is the remains of a dead star, which is called a white dwarf. White dwarfs are extremely dense. They have a mass similar to that of the Sun, but compressed into a ball the size of the Earth. The two stars orbit each other like the Earth and the Moon. The white dwarf has a gravitational field that is so strong it sucks material from its companion star. The material falls towards the compact star and is heated up to millions of degrees, so that it emits strongly in X-ray light.
The majority of stars in our Universe are in binary or multiple systems, that orbit about their common centre of mass. When one of the stars dies, the outer layers of the stars are blown off into Space and only the central core of the star remain. This core, the white dwarf, has an intense gravitational field. The field is so strong, it rips material from the companion star. This material falls towards the white dwarf. If the white dwarf magnetic field is sufficiently weak, the matter forms an accretion disc around the white dwarf. But if the magnetic field is stronger, the matter latches onto the magnetic field and follows it until it impacts the polar regions of the white dwarf. As it impacts, bright ultra-violet and X-ray emission is generated. This kind of cataclysmic variable is called a "polar".