How does diffraction grating create a spectrum?

no idea but there is a cool website called wikianswers you might be able to ask on

A diffraction Grating is an array of arranged lines, normally a wavelength apart. They are commonly used to measure the size of your penis because its so small it has to be measured in nanometers.

Prisms create spectrum of colors by the cause of refraction.

Try Edmund Scientific

diffraction grating is a lik a plate on which many slits are present....light undergoes diffraction through the slits...diffraction grating then splits light into its conctituent colours which appear...