Later Vedic Texts--The Brahmanas, Aranyakas, and Upanisads
In the beginning, the Samhitas were chanted to the gods as the offerings were being burned, as an invitation for the gods to come and bless them. In time (and we infer this from the changing nature of the texts themselves) the emphasis shifted from the power of the gods to the power of the sacrifice--the conviction that if one performed the actions and chanted all the formulae exactly right (and there was a priest whose sole function was to correct other people's mistakes), that this would guarantee one success. In this view, the real power lay not in the gods, but in the ritual actions themselves.
Not surprisingly, the texts from this time, the Brahmanas, are sacrificial manuals. They detail every possible aspect of the sacrifice, from the configuration of bricks for the altar, to the materials to be used for sacrifice, to the amount of grass one should strew on the ground, to the size of the spoon used to ladle clarified butter into the fire, etc. The Brahmanas are dense ritual manuals, giving directions for these sacrifices in excruciating, nauseating detail; those who do not share this faith in sacrifice find the Brahmanas both difficult and tedious.
Somewhere about 800 BCE (by the best guess), the ritual-mechanical emphasis on sacrifice began to fade, as evidenced by the two final types of text--the Aranyakas ("Forest-Books") and the Upanishads ("Secret Teachings"). In both the emphasis shifts from the power of the sacrifice to speculation on the power behind the sacrifice, which made it efficacious. Given the name, it seems reasonable that the "forest-books" were composed by people who had renounced normal society to live in the forest (whereas the cult of sacrifice, with its highly trained priesthood and royal patronage, was clearly part of the social establishment).
The differing religious orientations found in these two types of texts mark the division between two different notions of the ideal religious figure. The cult of sacrifice stressed encyclopedic memorization of the sacred texts and the painstaking performance of the sacrificial rites--essentially a question of technical profiency over a large body of knowledge. It was a hereditary profession, in which sons gained the necessary training from their fathers, and the access to this training was restricted to a small group of people, who called themselves brahmanas (i.e., brahmins). Braahmanas tended to live in society as householders, since it is difficult to sustain a hereditary group unless it reproduces itself. Their religious practice was public, "traditional," and "establishment"--since their major patrons were the kings and nobles, the sacrifices performed tended to benefit these ruling groups, either addressing their personal concerns (e.g., no sons) or for the more general welfare of the kingdom.
The opposing model of religious life was not based on learning, memorization, and/or technical proficiency over a body of knowledge, but on an individual's personal insights and realization into the nature of the universe. Such sorts of essentially mystical insights depends more on an individual's efforts and innate capacities, rather than his/her education and family, and because of this predominant emphasis on individual effort, people on this path were called shramanas ("strivers"). Shramanas tended to lay far less emphasis on the importance of learning, hints of this can be seen in the selections from the Chandogya Upanishad in the following section. The Shramanas are believed to have lived outside society (as the name "forest-books" hints), and their religious practice is private, non-hereditary, and internalized. Given their individualized practice, and the stress on working with a teacher, the shramanas were a diverse group. Some groups accepted the Vedas as sacred texts, whereas others rejected this notion (among them, the Buddhists and Jains). What these groups shared was not so much a body of ideas, but a commitment to a certain "style" of religious life. And is from the first such group, who saw themselves as working within the tradition of the Vedas, that we get the Upanishads.
The Aranyakas and Upanishads are about the search for first principles--searching for what makes the universe go, the power BEHIND all things which allows them to operate. They also attempt to discern the essential nature of both the universe and the human being. In the end, the essence of the universe is found in brahman, for which the best translations might be "Being" or "Reality," while that of the human being is called atman ("Self"). The essential insight of the Upanisads is the identity of brahman and atman, macrocosm and microcosm.
The Upanishads are profoundly speculative, and they are far from systematic--the answer that one receives in one place may be contradicted later in the same text, or in another Upanisad. Remember that these texts were not written, but oral, and thus hidden, esoteric knowledge (to gain access to them one not only had to find someone who knew them; one also had to convince that person to reveal it). Although they do give answers, the heart of the Upanishads lies in their questions, in the continual and committed effort to unravel the mysteries of the universe. We may dismiss their answers, but we cannot ignore their questions.