Among the many systems of classification of crops, commercial, agricultural, and taxonomical can be considered to be the most widely accepted agriculture classification of crops.
Commercial classification
Plants are classified according to commercial purposes as food crops, industrial crops, and food adjuncts.
Wheat and barley were generally the offerings - called Cerealia munera after Ceres, the Roman goddess of harvest. Subsequently, the grains used for food, especially for making bread were called Cerealia or cereals. The term is applicable to the grains obtained from the members of the family Poaceae, such as rice, wheat, maize, sorghum, barley, millet, rye, oats.
Pseudocereals are plants used similarly to cereals but belonging to families other than Poaceae such as buckwheat or amaranth.
Pulses
The term pulse is used for the seeds of plants from the Fabaceae family (legumes), such as beans, pea, lentil and chickpea. Pulses supply proteins and form chief source in vegetarian food. Leguminous plants fix nitrogen in root nodules - produced with the help of nitrogen fixing bacteria.
The grasses and legumes which are grown in arable land and left for animals to graze-on. The straw of paddy and cholam and dry plants of pulse crops and groundnut form important forages. The foliage of a number of trees and shrubs which are edible to animals form another source of forage especially in dry areas and during the periods of scarcity.
There are trees found in most parts of the world but mostly Africa that make or provide rubber for human uses.
Green manures and green-leaf manures
Growing of special crops for adding organic matter and nitrogen to the soil and by ploughing them in situ is called green manuring.
Sunhemp
Pillipesara
Kolingi
Indigo
Sesbania speciosa
Taxonomical classification
Taxonomical classification includes the taxonomical aspects of crops which is their morphology and economical parts and agrobotanical characters. This classification increases understanding of the morphological characters of any particular family.
As a disadvantage, this classification crops with different economic uses and morphological and other agrobotanical peculiarities when brought under one family do not generally bring out the economic importance of the individual crops.
References
1. Botany of Tropical Crops - Dr. V. Chellamuthu
2. Economic Botany by Kochhar