A mass spectrum is a graph of ion intensity as a function of mass-to-charge ratio.
Mass spectra are often depicted as simple histograms as shown in the figure on the left. This record of ions and their intensities serve to establish the molecular mass and structure of the compound being mass analysed.
For example, the figure on the left shows a mass spectrum of the simple molecule carbon dioxide, CO₂.

In this example, all the ions are positively charged. (It is possible to generate and detect negative ions as well.) The ionised CO₂ molecule (or molecular ion) appears at m/e = 44. Since the ionisation process breaks up or fragments some of the CO_2⁺ molecular ion, a fraction of the ions appear in the spectrum at m/e values less than the m/e value that corresponds to the molecular mass of CO₂.
Cleavage of a carbon-oxygen bond in the molecular ion to produce ionised carbon monoxide or ionised atomic oxygen result in the fragment ions at m/e = 28 and 16 respectively.
Loss of two neutral oxygen atoms results in an additional fragment at m/e = 12 for ionised carbon. The molecular ion is designated as M⁺ or CO₂⁺ and the fragment ions are designated as CO⁺, O⁺ and C⁺.