A grating (or diffraction grating) is a kind of optical device that defracts light, which bends it, the amount of the angle depending upon the wavelength. As such, it separates the light into colors or wavelengths to make an instrument that measures intensity by wavelength, a grating spectrometer.
Gratings can be transmissive or reflective and are sometimes curved. They can be designed for X-ray to infrared radiation.
An echelle grating is a particular kind of reflective grating with a low density of grooves, typically used as one of a pair of reflective gratings, in an echelle spectrometer. HARPS has one.
A blazed grating or echelette grating is optimized (blazed) for one wavelength.
A Littrow configuration is such that the blaze angle and incidence angle are identical, and in a reflection grating, the diffracted beam is in the direction of the incident beam.
A volume phase holographic grating (VPH grating) uses parallel strips with varying refractive index, embedded in gel between glass plates. The pattern is formed using laser diffraction patterns on a photosensitive gel.