The Upanisads come at the close of the Aranyakas. If the Samhita is the tree, the Brahmana the flower and the Aranyaka the fruit (i. e. in its unripe stage), the Upanishads are the mellow fruit - the final fruit or "phala". The Upanisads are to the seeker the direct means of realising the non-difference between the jivatman (individual self) and the Paramatman. The purpose of the Samhita and the Aranyaka is to take us to this path of knowledge. Though a number of deities are mentioned here and there in the Upanisads, the chief objective of these texts is inquiry into the Ultimate Reality and the attainment of the stage in which one becomes wise enough and mature enough to sever oneself from all karma. It is on this basis that the Vedas are divided into the karmakanda and the jnanakanda, the part dealing with works and the part dealing with knowledge [enlightenment]. The two are also spoken of as the Purvamimamsa and the Uttaramimamsa respectively.
The great sage Jaimini's sastra based on his inquiry into the karmakanda is called Purvamimamsa. His teaching is that the karmakanda, constituting the Vedic rites and duties, is itself the final fruit of the scripture. Similiarly, Vyasa has in his work, the Brahmasutra, inquired into the jnanakanda and come to the conclusion that it represents the ultimate purpose of the Vedas. The Upanisadic jnanakanda is small compared to the karmakanda. The Jaiminisutra has a thousand sections ("sahasradhikarani"), while Vyasa's Brahmasutra has only 192 sections. Just as the leaves of a tree far outnumber its flowers and fruits, in the case of the Vedic tree the karmakanda is far bigger than the jnanakanda.
In other countries philosophers try to apprehend the Truth on an intellectual plane. The Upanisadic inquiry is differnt, its purpose being to realise inwardly the Truth perceived by the mind or the intellect. Is it enough to know that halva is sweet? You must ecperience its sweetness by eating it. How are the Upanisads different from other philosophical systems? They (the Upanisads) consist of mantras, sacred syllables, and their sound is instinct with power. This power transforms the truths propounded by them into an inward reality. The philosophical systems of other countries do not go beyond making an intellectual inquiry. Here, in the Vedas - in the karmakanda - a way of life is prescribed for the seeker with actions and duties calculated to discipline and purify him. After leading such a life and eventually forsaking all action, all Vedic karma, he meditates on the truths of the Upanisads. Instead of being mere ideas of intellectual perception, these truths will then become a living reality. The highest of these truths is that there is no differnce between the individual self and the Brahman.
It is to attain this highest of states in which the individual self dissolves inseperably in the Brahman that a man becomes a sannyasin after forsaking the very karma that gives him inward maturity. When he is initiated into sannyasa he is taught four mantras, the four [principal] mahakavyas. The four proclaim the identity of the individual self (jivatman) with the Brahman. When these mahavakyas are reflected upon through the method known as "nididhyasana", the seeker will arrive at the stage of realising the oneness of the individual self and the Brahman. The four mahavakyas occur in four differnt Upanisads. Many are the rites that you have to perform, many are the prayers you have to recite and many are the ways of life you are enjoined to follow - all these according to the Samhitas and Brahmanas. But, when it comes to achieving the highest ideal, the supreme goal of man, you have no alternative to the Upanisads and their mahavakyas.
"The Brahman means realising the jnana that is the highest" (Prajnanam Brahma): this mahavakya occurs in the Aitareya Upanisad of the Rgveda. "I am the Brahman" (Aham Brahmasmi) is the mahavakya belonging to the Brhadaranyaka Upanisad of the Yajurveda. "That thou art" or "the Paramatman and you are the one and the same" (Tat tvam asi) is from the Chandogya Upanisad of the samaveda. THe fourth mahavakya, "This Self is the Brahman" (Ayam Atma Brahma), is from the Mandukya Upanisad of the Atharvaveda.
In his Sopana Pancaka, which contains the sum and the substance of his teachings, the Acharya urges us to chant the Samhitas (of the Vedas), perform the duties laid down in the Brahmanas and, finally, to meditate on the mahavakyas after recieving initiation into them, the purpose being our oneing with the Brahman.
The Vedas find their final expression in the Upanisads. Indeed, the Upanisads are called "Vedanta". They form the final part of the Vedas in two ways. In each recension we have first the Samhita, then the Brahmana which is followed by the Aranyaka, the Upanisad coming at the close of the last-mentioned. The Upanisads throw light on the meaning and the purpose of the Vedas and represent the end of the scripture in more than one sense: while their text forms the concluding part of the Vedas, their meaning represents the Ultimmate Truth of the same. A village or town has a temple; the temple has its gopuram; and the gopuram has a sikhara over it. The Upanisads are the sikhara, the summit, of our philosophical [and metaphysical] system.
"Upa-ni-sad" means to "sit near by". The Upanisads are the teachings imparted by a guru to his student sitting by his side [sitting at his feet]. You could also take the term to mean "that which takes one to the Brahman". "Upanayana" may be interpreted in two ways: leading a child to his guru; or leading him to the Brahman. Similiarly, the term Upanisad could also be understood in the above two senses.
If a student sits close to the teacher when he is recieving instruction it means that a "rahasya" (a secret or a mystery) is being conveyed to him. Such teachings are not meant to be imparted to those who are not sufficiently mature and who are not capable of cherishing their value. That is why in the Upanisads themselves these words occur where subtle and esoteric truths are expounded:"This is Upanisat. This is Upanisat". What is held to be a secret in the Vedas is called a "rahasya". In the Upanisads the term "Upanisat" is itself used to mean the same.
(see Chapter 33 of this part, entitled "The Ten Upanisads")
For a general background, please see here