Our Sun is a single star, but many stars exist in binary systems consisting of two or more stars in orbit around each other (or more accurately, around their common centre of mass).
There are different types of binary star – visual, spectroscopic and eclipsing:
A visual binary is one that can be distinguished as two separate stars using a telescope.
A spectroscopic binary is identified from the spectrum. Over time the system shows a spectrum that oscillates, being doppler shifted towards the blue and red with a regular period.
An eclipsing binary is one in which one stop passes in front of the other, partially preventing light from reaching us. If the two stars are the same size and equal brightness, then periodically the brightness of the system will drop by 50%.