A binary star system consists of two stars orbiting around a common center of mass. The system consists of a bright star known as the primary and a second one referred to as a companion star or a secondary. Studies conducted in 19th century showed that most stars are part of either a binary star system or other systems that comprise of more than two stars.
Binary systems are considered very important in astrophysics since calculations of the orbits enable the masses of their components to be determined directly. This is turn allows other stella parameters such as density and radius to be indirectly estimated. Basically there are 4 types of binary stars, these include:
Visual Binary Stars
The visual binary comprises of stars that can be resolved individually using a telescope. Long term observations are then made to map out the position of individual members of the binary system. With time the data is built up and used to map out the various orbits of the stars.
Visual systems tend to be stars that are close to earth. They are made up of stars that are separated by 10s to a few 100s of AUs. These stars are gravitationally drawn to each other although they do not interact unlike other binaries where stars may draw material from the surface of other stars in the system.
Eclipsing Binary Stars
Eclipsing binaries consists of stars whose orbit plane lies so close in the line of sight of the observer that the component undergoes mutual eclipses. Algol is the most popular example of an eclipsing binary.
Eclipsing systems are considered as variable stars, this is not because the light of the individual stars vary but because of the eclipse. The light curve from an eclipsing binary is characterized by moments of constant light followed by periodic drops in intensity.
Astrometric Binary Stars
These stars when observed for an extended period of time demonstrate a wobble effect in their motion. When the occurrence is periodic it shows that the wobbling happens due to gravitational pull of an invisible companion. Binary systems are usually detected by astrometric means known as astrometric binaries. Currently only a handful of binary systems have been detected astrometrically.
This is basically as the result of requirement for long term observations, position uncertainty and proper motion measurement. This problem is likely to be resolved by next generation space-based astrometric exploration. Sirius is currently one of the popular astrometric binary identified by Friendrich Bessel who observed wobbling in its motion.
Spectroscopic Binary Stars
These are systems whose evidence of existence comes from the Doppler effects on their emitted light. The system comprises of stars whose spectral lines in the light emitted from individual star shifts towards blue, then red as each move towards the earth then shifts away. This is accomplished in the course of its motion on its center of mass within the period of the common orbit. The spectroscopic systems considered among the 4 types of binary stars usually have stars that have very little separation; however their orbital velocity is usually very high.
There you have the 4 types of binary star systems. If I missed anything, let me know in the comments.