Generally, the light we see is composed of a mixture of wavelengths. White light is composed of red, green, yellow, and blue photons. We can separate light into its component wavelengths by using a dispersing element, either a prism or a diffraction grating. Once the light is dispersed, it forms a spectrum. A rainbow is an example of a spectrum (the dispersing elements are water droplets which act as prisms). The plural of "spectrum" is "spectra".

There are 2 basic kinds of spectra:

Continuous spectra, like a rainbow.
Line spectra. The light is concentrated into specific colors or wavelengths.

Spectra are a powerful tool for studying astronomical objects.

A diffraction grating consists usually of thousands of narrow, closely spaced parallel slits (or grooves). These grooves selectively filter the light, spreading it into patches of specific wavelengths, so that the resulting image beyond the grating contains the color bars representing the signature pattern of the source of light. Elements which are gaseous can be identified by shining a light through them. As long as the light source has a continuous spectrum, and the gas is cooler than the source, then the image created by the diffraction grating identifies the gas by its absorption lines.

Elements that are liquid or solid can be identified by examining the reflected light from a source with a continuous spectrum...

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