Hindu sacred scriptures
The śruti (= phonically transmitted revelation) or Veda

It comprises 4 Vedas:
The Rig-Veda (Veda of stanzas in praise of the gods)
The Yajur-Veda (V. of ritual formulas)
The Sâma-Veda (V. of songs and melodies)
Atharva-Veda (V. of Atharvan)
Each of the four Vedas comprises in turn:
A basic collection, the samhitâ (mostly in verse), called mantras when assigned to rites.
A prose explanation: the Brâhmanas
Another, more esoteric series: the Âranyakas
Speculative treatises: the Upanishads

The Samhitâ-s
The Rig-veda-samhitâ is the basic collection from which the others are derived. It comprises 1028 hymns divided into 10462 stanzas, the first of which is dedicated to Agni, patron saint of the Rig-Veda. The place of the RVS hymns is comparable to that of the Psalms in the Christian liturgy. They constitute a poetic treasure trove from which liturgical prayers and recitations can be drawn for a wide variety of occasions (see also here)
The Sâma-veda-samhitâ consists mainly of stanzas taken from the RVS and adapted for sung recitation. It is the canticle with musical notations and melody indications (see also here)
The Yajur-veda-samhitâ is a collection of mixed verse and prose formulas, directly dedicated to worship and arranged in the order in which they are used in ceremonies (see also here)
The Atharva-veda-samhitâ contains magical charms for long life, against illness, demonic possession, to win the love of others or wealth.

The Bhagavad-gita is a dialogue between Sri Krsna, God, the Supreme Person, and Arjuna, His devotee, close friend and disciple. Arjuna questions Krsna, who replies by explaining the science of spiritual realization.
The Bhagavad-gita is part of the Mahabharata, compiled by Srila Vyasadeva, the avatara-writer who, like Sri Krsna, appeared on Earth 5,000 years ago to write down Vedic wisdom for the benefit of future generations.
Universally recognized as the jewel of India's spiritual wisdom.
The Vedānta defines the nature of Existence, teaching that the Self (ātman) is of the same nature as Brahman, the ultimate undifferentiated Reality. Perception of this reality is obscured in man by erroneous knowledge (viparyaya) about himself and the world, which prevents him from experiencing the fullness of unity. In the Upaniṣad, pure Consciousness, called Brahman (the universal Self), is presented as the substratum of the universe, from which appear the world and also individualized consciousness (ahaṃkāra). But all these forms, according to the Vedānta, are merely illusory appearances, because only Brahman exists in reality. The whole world is not what it seems: it has no independent existence, it is the manifestation of an ultimate reality, it is a mere appearance, and it arises through the play of māyā, the creative power inherent in Brahman.
It is on this last aspect that the Advaita Vedānta, which is one of the most representative schools of Vedānta today, places particular emphasis (Ādi Śaṅkara, 800 A.D.) It is this that is at the origin of the concept of Non-Duality as it has spread throughout the world and mainly in the West. Shankara4 is said to have influenced many Indian thinkers such as Sri Aurobindo, Tagore, Osho, Ramana Maharshi and many foreign scientists such as Schrödinger and Einstein.
The other schools of Vedānta are: Viśiṣṭādvaita of Rāmānuja, Dvaitādvaita[ of Nimbarka, Dvaia of Madhva, Śuddhādvaita of Vallabha Acharya, Bhedābheda of Bhāskara.
This path to absolute knowledge taught in the Upaniṣad can be summarized by these three statements:
only brahman is real ( brahma satyam )
the world is illusory (jagan mithyā)
the individual is no different from brahman (jîvo brahmaiva nāparah).
The student must realize that the Self within him is no different from brahman, through direct experience: the Ātman is Brahman, which leads him to perceive the presence of brahman in everything.
It is the master (guru) who helps the student realize his eternal nature by delivering the teaching, summed up in this great statement:
Tat tvam asi : "You are That".
Having received the teaching, the student must reflect for himself, and he must also meditate on the Self. Contemplation on the message of the Scriptures is a support for this meditation, like this mantra for example, which affirms our divine nature and full of bliss:
So'ham: "I am He".
For the Advaita Vedānta, the universe is a single entity, an interconnected Totality. Distinctions between objects result from ignorance of the true nature of Reality, identical to brahman, which transcends time and space. In this state of ignorance, the individual is a prisoner of the world's illusions, and cannot escape successive reincarnations, the fruit of his karma.