This page last modified 1998 July 21
The term covers any star in which a property (usually brightness or colour) varies with time.
Eclipsing Binaries: the variation is caused by light from one star being eclipsed by the other member of the pair. Algol is the famous example.
Cataclysmic Variables: a binary system in which one member is a white dwarf. The build-up of material in the accretion disk causes unpredictable outbursts of energy. The most common case is that the star brightens by a few magnitudes, fading away after a few days, with the pattern repeating every few weeks or months.
Recurrent Novae: (see Novae, above)
Pulsating Variables: stars which change in luminosity as they pulsate regularly in and out (pulsars are not pulsating variables!). They include the RR Lyrae and Cepheid variables. The spectrum of the star pulsates in the same rhythm as the brightness. Mira Stars (e.g. omicron Ceti, a.k.a. Mira) are the most common pulsating variables; they are pulsating red giants and have periods of several hundred days.
David Levy's Observing Variable Stars is a good introduction to this fascinating branch of astronomy, where the amateur still makes a valuable contribution. It covers the nature of variables, as well as their observation, and has good charts and seasonal observing suggestions.