Members of one major grammatical category can be derived from those of the same or another category by the addition of suffixes to a stem.
For example, Nouns can be derived from Verbs and vice-versa. Nouns can also be derived from Nouns.
Derivational morphology deals with the above mentioned derivations and causes a massive production for vocabulary of the languages.
Reference: Asher R E, T C Kumari - Malayalam. Page 382.