The Vedas -- Rgveda, Yajurveda, Samaveda and Atharvanaveda -- are the first four of the pramanas (authoritative texts) of our religion and also the most important. Of the remaining ten, six are Angas of the Vedas and four are Upangas.
Man possesses a number of angas or limbs. In the same way the Vedas personified -- the Vedapurusa -- has six limbs. ( It must be noted that the Vedas are also spoken of as Vedamatha, Mother Veda. ) The four Upangas, though not integral to the Vedas, are supporting limbs of the Vedapurusa. The Angas, as already stated, are six in number -- Siksa, Vyakarana, Chandas, Nirukta, Jyotisa and Kalpa. The four Upangas are Mimamsa, Nyaya, Purana and Dharmasastra.
The Vedas are fundamental importance; the Angas and Upangas derive their importance from them. Ayurveda, Dhanurveda, Arthasasthra and Gandharvaveda are called Upavedas, subsidiary Vedas. Their connection with the prime scripture is thus obvious.
The Vedas must be learned along with the Angas and Upangas. Such a thourough study of the scripture is called "Sa-Anga-Upanga-adhyayana" (study of the Vedas with the Angas and Upangas). The term "sangopanga", which has come into popular usage, is derived from this. If a speaker deals with a subject thoroughly, whether it be politics or something else, we use the word "sangopanga" in describing his performance. The term refers to the ancient caturdasa-vidya (the six Angas plus the four upangas). We have totally forgotten the old system of education but our culture is so steeped in it that we still use the term (sangopanga) to refer to any full scale treatment or exposition of a subject. The inference is clear. That for centuries the Vedas, together with their Angas and Upangas formed such an intimate part of life in Tamil land that a term associated with this tradition, "sangopanga", is still used by the common people there. But the irony of it is that today we do not know even the names of these old sastras.
The Vedas form the core of our religion and are the direct authority for our dharma and for all our religious practices. They are our Bible, our Qur'"an, our Granth sahib. But, of course, the Vedas are far far older than these scriptures of other faiths. All of them originate from truths found in the Vedas. The very word "Veda" connotes what is authoritative. There is a practice of reffering to the Bible, the Quran and other scriptures as the "Christian Veda", "Mohammedan Veda", "Parsi Veda", "Sikh Veda" and so on. Christians in India refer to the Bible as "Satya-Veda".
It is rather difficult to speak about the Vedas as a topic. One does not know where to begin and how to conclude. It is a bewildering task. The magnitude of our scripture is such -- and such is its glory.
"Pramanam Vedasca", says the Apastamba Dharmasutra. The Vedas are indeed the sources of all dharmas as well as the authority on which they are founded. A book that has been cherished by the great men of th Tamil country from the earliest times is Manu-dharma-nul (Manusmriti). Throughout India, Manu's dharmasastra is held in the highest esteem. In Tamil Nadu there was a king who earned the name of "Manu-niti-kanda-Cola" for the exemplary manner in which he administered justice. Once a calf got crushed under the wheel of the chariot ridden by his son. The king was so fair and strict that, when the aggrieved cow, the mother of the calf, sought justice, he ordered his son to be crished to death under the wheel of the same chariot. For us "Manu-niti-sastra"(Manusmriti) is the authority on dharma. But does it claim that it is the authority for all dharma? No. "Vedo'khilo dharmamulam", says Manu, i. e. the Vedas constitute the root of all dharma. They prescribe the dharma for all time, he says.
We must obey the dictates of the Vedas. When we are asked to accept a statement without questioning it, it is customary to remark; "Is that the word of the Vedas? " This confirms the fact that the common people believe that the word of the Vedas, or their injunction, must be obeyed without being questioned. The "Vedavak" (the word or pronouncement of the Vedas) has been our inviolable law for thousands of years.
For a general background, please see here