Algol Variables, or Algol-type binaries are a specific type of eclipsing binaries. It's a pair of stars, one brighter than the other, that eclipse one another when viewed from earth, and appear as a single variable star to the naked eye. When neither star is in eclipse, they appear as a single bright star. When the hotter star passes behind the cooler one, the star seems to get dimmer. When the hotter star passes in front of the cooler one, they appear dimmer than normal as well, but not as dim as when the hotter star is hidden.
Over 3,000 Algol star pairs have been documented and identified in the sky, but the category is named for the famous "Demon Star" Algol.