Cannabis sativa is the Species of the Genus Cannabis and includes two subspecies Cannabis sativa Indica and Cannabis sativa Sativa. In general Sativas tend to grow taller and have narrower leaves. While Indicas are shorter and have fat leaves.
Cannabis sativa Sativa
Cannabis sativa Indica
There also exists a species of the genus cannabis known as c. ruderalis, which has no therapeutic or recreational use. However, c. ruderalis has been bread with indica and sativa strains to produce hybrids which have active cannabinoids and they will flower without regard to the amount of light received. The resulting hybrids are known as autoflower strains and are popular with novice growers.