Nirukta is in reality a Vedic commentary or etymology and exegesis of the more difficult words of the Vedas, their origin, derivation and interpretation, with a view to a better understanding and exposition of the Vedic texts is called Nirukta and is regarded as one of the Vedangas. Its bases are the Nighantus, or lexicons collection of rare or obscure vedic words and synonyms arranged for the use of teachers. Yaska had before him five such collections. The first three contain group of synonyms, the fourth specially difficult words, and the fifth a classification of the vedic gods. These Yaska explained for the most part in the twelve books of his commentary. In so doing he adduces as examples a large number of verses, chiefly from the Rigveda which he interprets with many etymological remarks.
The first book is an introduction, dealing with the principles of grammar and exegesis. The second and third elucidate certain points in the synonymous Nighantus; Books IV — VI comment on the fourth section, and VII — XII on the fifth. The Nirukta, besides being very important from the point of view of exegesis and grammar, is highly interesting as the earliest specimen of Sanskrit prose of the classical type, considerably earlier than Panini himself. Yaska already uses essentially the same grammatical terminology as Panini, employing for instance, the same words for root (dhatu), primary and secondary suffixes.
One point of very great importance proved by Nirukta is that the Rigveda had a very fixed form in Yaska's time. Yaska explains the meanings of words and investigates their origins, illustrating his theme with Vedic references.