Spectroscopy is the study of variation of a particular physical quantity of a system with respect to the frequency. This frequency could be the frequency of light which is made to interact with the system or the frequency of light emitted by the system. The frequency that is made to interact with the system in spectroscopy could be absorbed by the system or scattered by the system.
Spectroscopy can be used to study distant objects or astronomical objects. For example, by observing the light emitted by the sun, we can determine it's composition or physical nature through spectroscopy. By the phenomenon of Doppler Shift, one can also measure the velocity of astronomical objects through spectrometry.
Spectroscopy is used in chemistry to identify the chemical properties of substances, or for the identification of substances in analytical chemistry. This is done by observing the spectrum of the substance, which can be either the emission spectrum or the absorption spectrum.
The method of spectroscopy needs a coherent and near monochromatic source of light. Because of this, lasers find use in spectroscopy.