As the name suggests these are two stars orbiting around a common center of mass. Binary star is the most common type of multiple star system. More than four-fifth of the light that we see are part of the multiple star system. One could be brighter than the other or both can have the same brightness. Binary stars provide information about the mass of a star, the size, the temperature and the radius. These stars are classified based on their orbits, wide, close, visual, spectroscopic, eclipsing and astrometric binaries.
Wide binaries are the stars that evolve on the same orbit yet are spread apart from each other and will have very little impact on each other. Close binaries evolve nearby,being able to transfer their mass from one to another and at times pull in the smaller star completely. The visual binary classification is made based on how they are obseved. These stars are wide enough that they can be obseved through a telescope or a binocular. Five-ten of the visible stars are part of the visual binaries.
Spectroscopic binaries are classified so if their emit a certain wavelength. When viewed through a telescope, they appear to be close. Eclipsing binaries will have an orbit that are at an angle based on the line of sight from the Earth. This angled orbit will make it look as though is passing infrornt of the other causing it to look like an eclipse. Astrometric binaries are ones that are too dim to be seen and thus can only be infered as they seem to be dancing around in an empty space.