Veda means knowledge, and the four Vedas are counted as shruti — that which was heard by the rishis rather than composed by them. Each Veda opens with its Samhita of hymns and continues through the Brahmanas, Aranyakas, and Upanishads, so a single Veda holds both ritual and contemplation within it. Tradition holds that Sage Veda Vyasa arranged the received hymns into these four collections.
The four Mahavakyas take one great statement from each Veda — Prajnanam Brahma, Aham Brahmasmi, Tat Tvam Asi, Ayam Atma Brahma. Four Vedas, four sentences, one conclusion.
Source: Aitareya 3.3; Brihadaranyaka 1.4.10; Chandogya 6.8.7; Mandukya 1.2Rigveda ऋग्वेदः
The Rigveda gathers its hymns into ten mandalas, of which the family books are attributed to particular rishi lineages. Its suktas include the Nasadiya Sukta on the origin of things, the Purusha Sukta, and the verse to Savitr that came to be recited as the Gayatri. The Dana Sukta of its tenth mandala speaks directly of giving.
Yajurveda यजुर्वेदः
The Yajurveda carries the formulas spoken by the priest during yajna, so its verses are arranged for use in the rite rather than for recitation alone. It comes down in two recensions, the Shukla and the Krishna Yajurveda, and the Taittiriya Samhita belongs to the latter. The Shatapatha Brahmana, attached to the Shukla recension, expands on the meaning of the rites it accompanies.
Samaveda सामवेदः
The Samaveda sets its verses to melody, and the saman is the sung form in which they are offered. Its collections are arranged for the udgatr, the priest whose office is the singing itself. The chants of the Samaveda are counted among the roots of the Indian musical tradition.
Atharvaveda अथर्ववेदः
The Atharvaveda takes its name from the Rishi Atharvan and turns toward the life close at hand — healing, the household, and the ordering of daily things. Its Prithvi Sukta addresses the earth as mother and asks her forbearance of those who walk upon her. The Mundaka, Mandukya, and Prashna Upanishads are attached to this Veda.