A genus refers to a group of plants which share certain structural characteristics. The genus name may come from mythology, literature, people, places, or something the plant resembles. The species name usually refers to a place where the plant is native, the plant's appearance, or the name of the person credited with discovering it. Individual species are precisely identified by analyzing the flowers and seeds of each plant. The determining criteria might be the number of petals on the flower, or a certain characteristic of the seed.
Varieties are subdivisions of species.
They refer to naturally occurring changes or mutations which create a distinctively different plant in appearance. This often refers to flower color, or variations in growing habits. Cultivars refers to varieties which, although they occurred naturally, can only be replicated by asexual propagation and human intervention by cloning. Hybrid refers to new varieties of plants, which were created by humans through cross pollination of separate varieties. The naming of hybrids is usually done by the creator of the hybrid, and is proceeded by an 'x'.