Vedas are the fundamentals on which the sanatana dharma is based on. Vedas are also known as Sruthi or Amnaya. Sruthi means that which is heard. In the ancient times, the vedas which originated by itself (swayambhu) let themselves to be heard by the rishis who were doing penance. Thus, even those rishis are not the authors of the vedas.
There are four Vedas: the Rigveda, the Yajurveda, the Samaveda and the Atharvaveda. Each Veda has been subclassified into four major text types – the Samhitas (mantras), the Aranyakas (rituals), the Brahmanas (commentaries), and the Upanishads (philosophy).
In the olden days, the vedas were transmitted through oral tradition unlike the present days in which there are written manuscripts. Intonations are very important while reciting the vedas as different tones may attribute different meanings to the same mantra. The upward, downward and median tones (swaras) are termed udaaatta, anudaatta and swaritha respectively. To elucidate the point, the story of Vritrasura is explained. His father, Twashta prayed for a son who will kill Indra. But his error in pronouncing the mantras led to his son being killed by Indra. Many teachers of the vedic period have mentioned about the futility of reciting veda mantras without knowing their meaning.
To understand the depth of vedic hymns, it is imperative to know the meaning of shiksha, vyakarana, chandas, niruktam, jyothisham and kalpam. These 6 are known as vedangas. Shikha, varna, swara, maatra, balam, saama and santhaaana are needed for proper utterance of vedic mantras. The 4 vedas, 6 vedangas along with puranas, nyayas, dharmashastras and meemamsas are together known as vidyas which are 14 in total.
The vedas are also known as thrayee which means three. In the olden days, only rig, yajur and sama vedas were present. Rig veda mantras when sung with proper intonation becomes sama veda. Atharvana veda originated after the thrayee. In the olden days, there were people well versed in all the vedas and were known as Trivedis or Dwivedis depending on the number of vedas they are well versed in.