A spectrograph is a spectrometer that images a range of wave-lengths simultaneously, either onto photographic film or a series of detector elements, or through several exit slits (sometimes called a polychromator). The defining characteristic of a spectrograph is that an entire section of the spectrum is recorded at once.
Plane grating monochromator mounts have an exit slit through which a narrow spectral region passes; the center wavelength of this spectra region is changed by rotating the grating. Alternatively, a wide spectral region can be imaged at once by leaving the grating fixed and using a series of exits slits (or an array of detector elements) in a focal plane. Such optical systems are called spectrographs.
Often the imaging properties of a plane grating spectrograph (with no auxiliary optics) are acceptable over only a portion of the spectrum of interest, which requires the use of additional lenses or mirrors to provide additional focusing power to render the focal curve as close to the line (or curve) represented by the slits or detector array.
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