What's the difference between grism and grating?
Transmission gratings on their own introduce chromatic aberration.  This is because they change the effective focal length and do so as a function of wavelength.  The chromatic aberration can be eliminated by introducing a prism of the correct dispersion.  Known as a grism, the grating/prism combination provides an unaberrated image at the 0th order in addition to the spectrum, although the spectrum has fairly low resolution.