Types of Variables
Different objects in the sky have different brightnesses, as measured by their magnitude, or energy flux. Any object whose brightness appears to change significantly is called a variable. Variability has many possible causes, but we can divide them roughly into intrinsic and extrinsic types. One very informative (but fairly complex) graphic, the variability tree, illustrates the relationship between many different types of celestial variables.
If the actual luminosity (the amount of electromagnetic power) from a star is truly changing, it is called an intrinsic variable. Intrinsic variables can be roughly divided into pulsating, eruptive, or cataclysmic variables, although some rare variable stars show more than one such phenomenon, like the R Corona Borealis stars.
Sometimes, a star whose luminosity is steady appears to vary because of variable external factors that affect how much light reaches us from the star. Extrinsic variability is really only apparent variability, meaning that the object itself is not getting brighter or fainter overall, but its brightness appears to be changing because the object's orientation to us is changing, or some other object is intervening along our line of sight to the object.