Variable stars are stars that change in brightness over time. They can be classified into two main types: intrinsic and extrinsic variables. Intrinsic variables, like Cepheid variables, change their brightness due to internal processes, such as pulsations. These stars expand and contract, leading to periodic changes in luminosity. Extrinsic variables, on the other hand, change brightness due to external factors. A common example is eclipsing binaries, where two stars orbit each other, and one star passes in front of the other, blocking its light. This results in a regular pattern of brightness changes as seen from Earth.