So far, in speaking of the Vedas, I have dealt mainly with the Samhita part of each sakha or recension. We have already seen that the Samhitas are the main text of the Vedas. Apart from them, each sakha has a Brahmana and an Aranyaka.
The Brahmana lays down the various rites - karma - to be performed and explains the procedure for the same. It interprets the words of the mantras occuring in the Samhita, how they are to be understood in the conduct of sacrifices. The Brahmanas constitute a guide for the conduct of yajnas.
The word "Aranyaka" is derived from "aranya". You must have heard of places like "Dandakaranya" and "Vedaranya". "Aranya"means a "forest". Neither in the Samhita nor in the Brahmana is one urged to go and live in a forest. Vedic rites like sacrifices are to be preformed by the householder (grhastha) living in a village. But after his mind is rendered pure through such rites, he goes to a forest as a recluse to engage himself in meditation. It is to qualify for this stage of vanaprastha, to become inwardly pure and mellow, that Vedic practices like sacrifices are to be followed.
The Aranyakas prepare one for one's stage in life as an anchorite. They expound the concepts inherent in the mantras of the Samhitas and the rites detailed in the Brahmanas. In other words, they explain the hidden meaning of the Vedas, their metaphorical passages. Indeed, they throw light on the esoteric message of our scripture. For the Aranyakas, more important than the performance of sacrifices awareness of their inner meaning and significance. According to present-day scholars, the Aranyakas incorporate the metaphorical passages representing the metaphysical inquires conducted by the inmates of forest hermitages.
The Brhadaranyaka Upanishad, as its very name suggests, is both an Aranyaka and an Upanishad, and it begins with a philosophical explanation of the horse sacrifice.
For a general background, please see here