The Magellanic Clouds are small irregular galaxies that can only be seen from the Southern Hemisphere. The Magellanic clouds are very close to us, only about 50-60 kpc away, so they have been very widely studied. Because they are so close to our Milky Way, the SMC in particular, is currently in the process of being ripped apart by tidal forces, and will eventually merge with the Milky Way. Studies of variable stars in both the LMC and SMC form the basis for distance calibrations to other galaxies much farther away. In general a variable star is any star that changes brightness over a relatively short period of time.
Extrinsic variables are geometric variables such as eclipsing binary systems, and rotating stars.
In these systems only the geometry of the two stars or the rotation of a star causes the variation in the light. Pulsating stars are those that undergo periodic expansion and contraction of their outer atmospheres. These include classes of stars such as Cepheids, RR Lyrae stars, RV Tauri stars, Delta Scuti stars, and Mira stars. They include Supernovae, Novae, and Dwarf Novae. There are other star types, such as T Tauri stars, flare stars, and magnetic stars, that do not fit nicely into either of these categories.