As already mentioned, a species is a group of plants from the wild which resemble each other very much and of which one expects that they all have the same ancestor. Plants that are raised and cultivated by men are called cultivars (cultivated varieties).
Cultivars can be placed in groups, just like wild plants. This is generally done when a lot of cultivars belong to one crop. Such groups for cultivated plants are called cultivar groups. Frequently one joins cultivars in a cultivar group, when they share the same ancestors. But that is not always the most useful classification for cultivars. Sometimes is it easier to base the classification in cultivar groups on characters and not on basis of common descent. Characters used for cultivar groups are for example flower form, sensitivity to diseases or the way of cultivation. Such a classification in cultivar groups may change or be extended if there is practice's need to.