Origin of the Vedas
The Vedas are probably the earliest documents of the human mind and is indeed difficult to say when the earliest portions of the Vedas came into existence. As the ancient Hindus seldom kept any historical record of their religious, literary and political realization, it is difficult to determine the period of the Vedas with precision. Historians provide us many guesses but none of them is free from ambiguity.

Who wrote the Vedas?
It is believed that humans did not compose the revered compositions of the Vedas, which were handed down through generations by the word of mouth from time immemorial. The general assumption is that the Vedic hymns were either taught by God to the sages or that they were revealed themselves to the sages who were the seers or “mantradrasta” of the hymns. The Vedas were mainly compiled by Vyasa Krishna Dwaipayana around the time of Lord Krishna (c. 1500 BC)

Classification of the Vedas
The Vedas are four: The Rig-Veda, the Sama Veda, the Yajur Veda and the Atharva Veda, the Rig Veda being the main. The four Vedas are collectively known as “Chathurveda, ” of which the first three Vedas viz., Rig Veda, Sama Veda and Yajur Veda agree in form, language and content. 

Structure of the Vedas
Each Veda consists of four parts – the Samhitas (hymns), the Brahmanas (rituals), the Aranyakas (theologies) and the Upanishads (philosophies). The collection of mantras or hymns is called the Samhita. The Brahmanas are ritualistic texts and include precepts and religious duties. Each Veda has several Brahmanas attached to it. The Upanishads form the concluding portions of the Veda and therefore called the “Vedanta” or the end of the Veda and contains the essence of Vedic teachings. The Upanishads and the Aranyakas are the concluding portions of the Brahmanas, which discuss philosophical problems. The Aryanyakas (forest texts) intend to serve as objects of meditation for ascetics who live in forests and deal with mysticism and symbolism.

The Mother of All Scriptures
Although the Vedas are seldom read or understood today, even by the devout, they no doubt form the bedrock of the universal religion or “Sanatana Dharma” that all Hindus follow. The Vedas have guided our religious direction for ages and will continue to do so for generations to come. And they will forever remain the most comprehensive and universal of all ancient scriptures.

The Rig Veda: The Book of Mantra
The Rig Veda is a collection of inspired songs or hymns and is a main source of information on the Rig Vedic civilization. It is the oldest book in any Indo-European language and contains the earliest form of all Sanskrit mantras that date back to 1500 B.C. - 1000 B.C. Some scholars date the Rig Veda as early as 12000 BC - 4000 B.C. The Rig-Vedic ‘samhita’ or collection of mantras consists of 1,017 hymns or ‘suktas’, covering about 10,600 stanzas, divided into eight ‘astakas’ each having eight ‘adhayayas’ or chapters, which are sub-divided into various groups. The hymns are the work of many authors or seers called ‘rishis’. There are seven primary seers identified: Atri, Kanwa,Vashistha, Vishwamitra, Jamadagni, Gotama and Bharadwaja. The rig Veda accounts in detail the social, religious, political and economic background of the Rig-Vedic civilization. Even though monotheism characterizes some of the hymns of Rig Veda, naturalistic polytheism and monism can be discerned in the religion of the hymns of Rig Veda. Click for Text of Rig Veda

The Sama Veda, Yajur Veda and Atharva Veda were compiled after the age of the Rig Veda and are ascribed to the Vedic period.

The Sama Veda: The Book of Song
The Sama Veda is purely a liturgical collection of melodies (‘saman’). The hymns in the Sama Veda, used as musical notes, were almost completely drawn from the Rig Veda and have no distinctive lessons of their own. Hence, its text is a reduced version of the Rig Veda. As Vedic Scholar David Frawley puts it, if the Rig Veda is the word, Sama Veda is the song or the meaning, if Rig Veda is the knowledge, Sama Veda is its realization, if Rig Veda is the wife, the Sama Veda is her husband. Click for Text of Sama Veda

The Yajur Veda: The Book of Ritual
The Yajur Veda is also a liturgical collection and was made to meet the demands of a ceremonial religion. The Yajur Veda practically served as a guidebook for the priests who execute sacrificial acts muttering simultaneously the prose prayers and the sacrificial formulae (‘yajus’). It is similar to ancient Egypt’s “Book of the Dead”. There are no less than six complete recessions of Yajur Veda - Madyandina, Kanva, Taittiriya, Kathaka, Maitrayani and Kapishthala. 
Click for Text of Yajur Veda

The Atharva Veda: The Book of Spell
The last of the Vedas, this is completely different from the other three Vedas and is next in importance to Rig-Veda with regard to history and sociology. A different spirit pervades this Veda. Its hymns are of a more diverse character than the Rig Veda and are also simpler in language. In fact, many scholars do not consider it part of the Vedas at all. The Atharva Veda consists of spells and charms prevalent at its time, and portrays a clearer picture of the Vedic society. 
Click for Text of Atharva Veda