The Rig Veda is the oldest of the Vedas. All the other Vedas are based upon it and consist to a large degree of various hymns from it. It consists of a thousand such hymns of different seers, each hymn averaging around ten verses. The Rig Veda is the oldest book in Sanskrit or any Indo-European language. Its date is debatable. Many great Yogis and scholars who have understood the astronomical references in the hymns, date the Rig Veda as before 4000 B.C., perhaps as early as 12,000. Modern western scholars tend to date it around 1500 B.C., though recent archeological finds in India (like Dwaraka) now appear to require a much earlier date. While the term Vedic is often given to any layer of the Vedic teachings including the Bhagavad Gita, technically it applies primarily to the Rig Veda.
The Rig Veda is the book of Mantra. It contains the oldest form of all the Sanskrit mantras. It is built around a science of sound which comprehends the meaning and power of each letter. Most aspects of Vedic science like the practice of yoga, meditation, mantra and Ayurveda can be found in the Rig Veda and still use many terms that come from it.
While originally several different versions or rescensions of the Rig Veda were said to exist, only one remains. Its form has been structured in several different ways to guarantee its authenticity and proper preservation through time.