Clover is the common name for plants in the genus Trifolium. The genus name refers to the fact that leaves are trifoliate (three leaves).
There are about 300 clover species. Some of these are cultivated as fodder crops (e.g. White clover, Red clover and Crimson clover).
Note that some plants in other genera are also called clovers (e.g. Melilotus, Medicago).