Answer
The Rig Veda, Sama Veda, Yajur Veda, and Atharva Veda — these are the four Vedas. They are also called Samhitas. They are filled with mantras. Most of their content consists of hymns praising the devatas and such material that cannot be classified as "jnana" (spiritual knowledge). There is a small amount of jnana as well, but very little.
Toward the concluding period of the composition of the Vedas came the Upanishads, in which spiritual knowledge was found. This is why they are called "Vedanta" (the end or culmination of the Vedas). The Upanishads are also referred to as the "Jnana Kanda" (the section of knowledge) of the Vedas.
The principal Upanishads are considered to be eleven, known as the Ekadasha Upanishads. After these ancient and authoritative Upanishads, numerous texts were composed under the name "Upanishad" up through the medieval period. According to a survey by the Government of India, their number reaches up to 200. Among these is even the "Allah Upanishad," commissioned by the Mughal emperor Akbar. This is precisely why, when speaking of Vedanta, only the ancient Ekadasha Upanishads are referenced. Traditionally, however, 108 Upanishads are recognized.
Among these eleven Upanishads are the Ishavasyopanishad, the Kathopanishad, the Prashna Upanishad, the Brihadaranyaka Upanishad, the Chhandogya Upanishad, the Shvetashvatara Upanishad, and others.
In these eleven Upanishads, there is a profound discussion of Brahman and Atman. Based on these, several philosophical systems were subsequently developed, collectively known as Vedanta Darshana. The most prominent among them is Shankara Vedanta, the philosophical system developed by Adi Shankaracharya on the basis of these eleven Upanishads. However, scholars have found that what is called Shankara Vedanta was actually originated by Shankaracharya's guru's guru, and Shankaracharya expanded upon it.