Historically, cavalry is divided into light and heavy cavalry. The difference between them is primarily how much armour is worn by the soldiers, and thus how powerful their mounts must be to sustain the burden.
The power of mobility given by mounted units were recognised in early history, but (arguably) cavalry became dominant on the battlefield only as stirrup?s were put into use. Cavalry became obsolete with the advent of modern vehicles with effective engines in the 20th century.
See also: Knight, Pike?, Military tactics