Many stars are part of multiple star systems where two or more stars are gravitationally bound together and orbit around each other. Where two stars are bound, we call these binary star systems or double stars. We also see trinary and higher-order star systems.
Some double stars can be distinguished visually. In these cases we can observe two distinct stars in close proximity. In other cases, the stars are too close together and cannot be "split" using our telescopes. We can sometimes tell if there are multiple stars by looking at the spectrum, or colors of the light from these stars. These "spectroscopic binaries" are distingushed by overlapping absorbtion lines in their spectra.
The elements of double stars often will have different colors or different brightness. This make the viewing of double stars rewarding because some are quite striking.