It seems we had countless Vedas which were last in the passage of time. And whatever available were not in any systematic format. It was left for Vyasa Maharshi who is supposed to have lived five thousand years ago to collect and organize the remaining Mantras into four books. It is the same Maharshi who wrote Mahabharatham, Puranas which contain Bhagawatham and also Brahma Sutras. And thus he is called Veda Vyasa, his original name being Krishna Dwaipayana. The four books of the Vedas are Rig Veda, Yajur Veda, Sama Veda and Adharvana Veda. Some of them are in poetic form, some in prose and the Sama Veda is musical. Whether in prose or otherwise, they are all called Mantras. The terms mantra is used only for Vedas and not for any other subsidiary books. And these mantras had swara or intonation for the ease of chanting and learning by heart. This was necessary to pass on the Mantras by word of mouth, without mistakes. For the same reason, Vedas are called Sruthi, which literally means what is heard. Veda mainly contains three parts, They are Poorva Bhaga also called Karma Kanda, Mantra bhaga, then Antha Bhaga also called Gnana Kanda. And the Veda Antha Bhaga when the word are combined becomes Vedanta. The Poorva Bhaga contains activity oriented Yagnas etc, Mantra Bhaga contained Stothras suktams and other prayers, and the Vedanta part contains the Ultimate Phylosophy of our religion and is also called Upanishads. and Thus ideals with religion and the ideals with philosophy, and the Mantras are meant for Bhakti or devotion and remain the common thread through out life. It is the opinion of our Gurus that one should start the life with Karma and fulfill all worldly desires, and in the process rfine his mind, and Then he can pursue the Gnanam from the Veda Anta Bhagam for Moksha or Liberation. Karma is incomplete without Gnanam, and Gnanam is impossible without Karma. All our scriptures are in the form of a dialogue between a sishya and a Guru, Gita itself being a dialogue between Srikrishna and Arjuna, which signifies that one has to learn this teaching through a Guru alone. The language being Sanskrit and the text itself being confusing, if one attempts to study by himself, it may prove contradictory and it may do harm rather than any good. Hari Om.