The Vedas are theistic in nature and emphasise to know and adore God and act according to his eternal laws. They possess all true knowledge, and all that which can be know through knowledge. The four Vedas have predominantly have four main themes, viz., knowledge, activity, worship and human conduct including scientific and management technology (Vigyanam).
They provide all primary and causatic knowledge about God, human beings, nature, their activity, and relationships. All sorts of knowledge in every field of human activities, physical sciences and technologies are dealt there in brief. The Vedas contain all basic material, ethical, and logical principles inspiring practical teachings and ideals for daily needs which can help human beings in elevating them to te supreme state, i.e, Moksha, individually and discharging social responsibilities.
Broadly speaking, there are two types of knowledge in the Vedas - PARA and APARA. PARA deals with the concepts of God, His manifestations of powers and spiritualism while APARA is concerned with wordly matters and pleasures - thus covering the entire field of knowledge.
Maharshi Vyaas in Mahabharata says "Whatever texts of different subjects are there in the world, they owe their origin to the Vedas". Rishi YagyaValkya in his smriti says "All shastras have originated from the eternal Vedas only, and no book is comparable to the Vedas". Manu in his smriti reiterates "The primary source of all knowledge of the past, present and future are from the Vedas. This knowledge is eternal, truthful and authentic"
Professor K V Paliwal has in his book "Vedas Introduced", in a very lucid style and language giving answers to the questions as below. The full book in scanned copy is available on the following link:- http://www.vedamu.org/PageViewerImage.aspx?DivId=1814
Here is an attempt to provide the following more clarifications on the Vedas:-
1. Language / Construction of the Vedas