A cataclysmic variable is a close binary system where a white dwarf is accreting matter from what is usually a main sequence star. Typical separations between the white dwarf and companion star are equivalent to the diameter of the Sun, and the orbital periods range from 1 to 12 hours.
Although they can be magnetic, the majority of cataclysmic variables discovered to date are non-magnetic. In their quiescent state, cataclysmic variables can be identified through their distinctive spectra, and systems which have never been observed in outburst are known as nova-like variables.
An outburst (nova) occurs when a quantity of material sufficient to initiate nuclear reactions is transfered from the companion star to the surface of the white dwarf. This is the origin of classical novae and recurrent novae. Alternatively dwarf novae originate from instabilities in the accretion disk, or sudden transfers of matter through the disk.
Cataclysmic variables have much in common with X-ray binaries, and are often brighter at X-ray and ultraviolet wavelengths than in the optical universe. The X-rays are produced by the extreme temperatures generated where the accretion disk hits the surface of the white dwarf.