Plant growth regulators are small, simple molecules having different compositions. Chemically they may be indole compounds (indole-3-acetic acid, IAA), adenine derivatives like N⁶- furfrylamino purine, kinetin), derivative of carotenoids (abscisssic acid, ABA), terpenes (giberellic acid, GA_{3) ,} gases (ethylene, C₂H₄). 

They can be divided into two main groups on the basis of the function they perform. 

i. One group is involved in growth promoting activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation etc. For example Auxin, cytokinin, gibberelins etc. 

ii. Hormones in the other group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth inhibiting activities such as dormancy and abscission. For example abscissic acid and ethylene.