Most stars occur in multiple systems. Estimates are 50-70% of stars occur in binary systems. Binary stars are classified according to how they are observed.
Visual binaries are systems where the component stars can be resolved in a telescope. Visual binary stars are relatively close to the earth, and are separated by up to several hundred astronomical units.
Spectroscopic binaries are distant systems which are resolved by the technique of Doppler shift of spectral lines.
Novae are binary star systems where a white dwarf accretes hydrogen rich material from a low mass companion.