In plain words
One name for the mantras is saṃhita. That is why all of these were named the Ṛgveda Saṃhita, the Yajurveda Saṃhita and the Sāmaveda Saṃhita. Of these, the Ṛgveda is made of praise of the deities. The ṛṣis of those days went on remembering these Ṛgveda mantras of praise with a boundless divine feeling. Then, the Yajurveda is made of mantras. Worship of the forms of the deities: the mantras of this Yajurveda they brought into rites such as yajña and yāga, and into good deeds such as charity and acts of Dharma.
Each Veda they divided into three parts. These they called the Brāhmaṇas, the Āraṇyakas and the Upaniṣads. In this mantra saṃhita there were whole mantras alone. In this Yajurveda Saṃhita and in this Brāhmaṇa there was not only a great deal of karmakāṇḍa; it also took up the path of following the yajñas of mantra, of teaching and of Jñāna, which are its practice. This Brāhmaṇa in turn grew under three kinds of names. The Taittirīya Brāhmaṇa, the Aittirīya Brāhmaṇa and the Śatapatha Brāhmaṇa. All these are of the form of mantra. Through such changes the dividing went on, only for the good of the land, the uplift of man and his welfare.
By rites such as yajña and yāga the welfare of the world comes about. Their power was this: that the rains would fall in season and people would live in plenty. Since all this was joined with karmakāṇḍa, through rites such as yajña and yāga they enjoyed fruits such as these: the rain falling, the land growing green with crops, the harvests growing, and through that a life passed with no want of food and drink.
The Āraṇyakas. When they had lived through the householder's stage, they went in solitude to the forests or the groves nearby and, as they contemplated God, they developed through mantras the feelings they had held in remembrance. These feelings, which rose in a surge of emotion, they set into mantras, and went on remembering and contemplating them without ceasing; that is how these came to be called Āraṇyaka. All the Āraṇyakas are made up for the most part of karmakāṇḍa. Only at the end does it take the shape of brahmakāṇḍa. This very brahmakāṇḍa has served the yajñas and yāgas.
The Yajurveda arose from the root yaj. It is wholly contemplation of the forms of the deities.