The Veda is organized into four divisions. The Rig Veda, Sama Veda, Yajur Veda, and Atharva Veda. Samhita is the first and customarily of the main part of the Veda. Samhita is not a teaching; it is a collection of mantras associated with a Rishi, and a Devata. And it is designed to produce a certain effect or give access to a certain kind of supra natural force; which is the core of Vedas.
Each of the Veda there are recensions / variations or Shakhas / parts; with slightly different collection of mantras / texts / traditions. Each Veda (recension therefore) has four parts (popularly believed by scholars like Shankara)
· the Samhitas (Mantra Samhita - meaning collection of Mantras) – Only mantras and benedictions
· the Aranyakas (text on rituals, ceremonies, sacrifices and symbolic-sacrifices) - Philosophical speculation about reality. Somewhat like metaphysics, but tending to Adhyatma
· the Brahmanas (commentaries on rituals, ceremonies and sacrifices) - literature investigating in principle the rituals included in the tradition.
· the Upanishads (text discussing meditation, philosophy and spiritual knowledge) - A more focused investigation into the very essence of the Aranyaka.
Vedas consists of Mantra Samhithas only, which believed to be Apourusheya - not of human origin but divine creation. Brahmanas and Aranyakas are basically the same and Upanishads are written by sages (ancient scholars) as interpretation of Vedas.
The Samhitas are sometimes identified as karma-khanda (कर्म खण्ड, action/ritual-related section), while the Upanishads are identified as jnana-khanda (ज्ञान खण्ड, knowledge/spirituality-related section). The Aranyakas and Brahmanas are variously classified, sometimes as the ceremonial karma-khanda, other times (or parts of them) as the jnana-khanda.
The Vedic Samhitas were chanted during ceremonies and rituals, and parts of it remain the oldest living part of Hindu tradition.
Samhita means "put together / joined / union / collection", and "a methodically rule-based combination of text or verses". Samhita is the most ancient among Vedic Texts consisting of mantras, hymns, prayers, litanies and benedictions – sacred sounds with or without literal meaning, as well as panegyrics, prayers, litanies and benedictions petitioning nature or Vedic deities.
A collective study of Vedas and later text suggests that the compendium of Samhitas and associated Vedic texts were far larger than currently available. However, most have been lost at some point or over a period of Indian history. Vedic Samhita refer to mathematically precise metrical archaic text of each of the Vedas (Rigveda, Yajurveda, Samaveda and Atharvaveda).
The whole of the Rigveda-Samhita is in form of verses, known as Rik. ‘Rik’ is the name given to those Mantras which are meant for the praise of the deities. Thus, the collection (Samhita) of Riks is known as Rigveda-Samhita. The Rigveda Samhita contains about 10552 Mantras, classified into ten books called Mandalas. [https://en.wikipedia.org/wiki/Shakha]
Yajurveda-Samhitas are the prayer-books for the Adhvaryu priest; for the purposes of sacrificial rituals. His works vary from the selection of a plot of land for the sacrificial altar down to offering oblations to the sacred fires.
The Samaveda Samhita is an independent collection (Samhita), yet it has taken many verses, a large number indeed, from the Samhita of Rigveda. Samaveda-Samhita is the songbook of the Udgata priest (singers during the Sacrifice, Yaga/Yajna).
The Atharvaveda Samhita contains hymns many of which were charms, magic spells and incantations meant to be pronounced by the person who seeks some benefit, or more often by a sorcerer who would say it on his or her behalf. The most frequent goal of these hymns charms and spells were long life of a loved one or recovery from some illness. In these cases, the affected would be given substances such as a plant (leaf, seed, root) and an amulet. Some magic spells were for soldiers going to war with the goal of defeating the enemy, others for anxious lovers seeking to remove rivals or to attract the lover who is less than interested, some for success at a sporting event, in economic activity, for bounty of cattle and crops, or removal of petty pest bothering a household. Some hymns were not about magic spells and charms, but prayer qua prayer and philosophical speculations.
Some post-Vedic Samhitas are named after the Rishi’s (authors) Ashtavakra Gita, Bhrigu Samhita, Brahma Samhita, Charaka Samhita, Deva Samhita, Garga Samhita, Gheranda Samhita, Kashyap Samhita, Shiva Samhita, Sushruta Samhita (a treatise on food and medicine), Yogayajnavalkya Samhita.