A serovar or serotype is a group of microorganisms or viruses based on the cell surface antigens. Serovars allow organisms to be classified at the sub-species level; an issue of particular importance in epidemiology.

Serovars may be established based on virulence factors, lipopolysaccharides in Gram-negative bacteria, presence of a exotoxin (pertussis toxin in Bordetella pertussis, for example), plasmids, phages, or other characteristic which differentiate two members of the same species. ^,

Salmonella, for example, has 1800–2000 serovars: Salmonella enterica Serovar Typhimurium, S. enterica Serovar Typhi, and S. enterica Serovar Dublin, to name a few.
Vibrio cholerae, which causes cholera, has 139 serotypes, based on cell antigens. Only two of them produce an enterotoxin and are pathogens: 0:1 and 0:139.

Historical

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Serotypes were discovered by Rebecca Lancefield in 1933.

References

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Speciation | Scientific classification

Sérovar