INTRODUCTION 
The Taittiriya Upanishad is a  part of the Krishna Yajur Veda and forms a part of the seventh, eighth and  ninth chapters of the Taittiriya Aranyaka. This Upanishad occupies a unique  place in Indian Philosophy because it explains the Reality both in direct as  well as indirect terms. Let us see what we mean by direct and indirect way of  explaining.
In the field of Upanishad  knowledge, the mode of communication is through lakshana vritti where communication is accomplished by indication  through different pointers. There are two types of such pointers: they are tatastha lakshanam and svaroopa lakshanam. For  example, consider the word Brahman. This word is not available as the meaning  of any other known word. It is not comparable to anything that we know. It does  not have any attributes. It has no Guna or attribute. It has no action.  Therefore, a word defining Brahman is not there. Simply trying to use many words  to define Brahman does not really help. The only way knowledge about Brahman  can be communicated is through lakshana  vritti, by indication through appropriate pointers. 
There are two types of such  pointers. They are tatastha lakshanam and svaroopa lakshanam. In the tatastha lakshanam Brahman is indicated  not by what It is, but how It is related to the things that we know. For  example, when we say"I do namaskar to That Brahman fromwhich this entire creation is  born", this statement does not tell what Brahman is. It onlytells that there is something called  Brahman and That is the cause for this creation. Here again, we must understand  that this creation is not an attribute of Brahman and Brahman stands  independent of creation. Such method of indicating Brahman is called tatastha lakshanam, (tatastha means standing apart).
On the other hand, in svaroopa lakshanam one indicates Brahman  as It is. For example, when the Upanishad says: satyam jnanam anantam brahma, Brahman is satyam - that which is never subject to change, is jnanam - that which is all knowledge  Itself, is anantam -that which is  Limitlessness Itself. All these three indicators point to the same ONE BRAHMAN.  Here the Upanishad indicates Brahman Itself, as It is, and this method of  indication is called svaroopa lakshanam. In  this method of indication, we do not talk about creation at all. Even though we  do not talk about creation as such, Brahman being anantam - Limitlessness Itself, there can be nothingindependent of Brahman which means all  names and forms in this creation are non-separate from Brahman even though  Brahman Itself remains independent of this creation which is subject to change  from time to time. In this way, we gain knowledge of both satyam and mithya and  their relationships.
In the Taittiriya Upanishad,  Brahman is introduced by both types of indicators, namely tatastha lakshanam and svaroopa lakshanam which is the extraordinary feature in the mode of  communication or dissemination used in this Upanishad.
Though comparatively short, the Taittiriya Upanishad is  one of the important Upanishads and is recited in many parts of India with  proper accent, intonation and dedication. This  Upanishad, like most other Vedic chantings, has a particular accent, mode of  recitation,(svara) which has come  down from generation to generation, by tradition (which also varies slightly  from region to region). It is regarded as a source-book of the Vedanta  philosophy. The topics covered are presented very methodically which are  enlightened further by Adi Sankaracharya’s commentary thereon. 
An interesting legend is closely  associated with the Taittiriya Upanishad. The legend goes that once saint  Vaisampayana got angry with one of his prominent disciples called Yajnavalkya.  As the guru had got angry with his disciple he demanded that his disciple  should give back all the knowledge which he has acquired from the guru. Being  ordered in such a manner from the guru, Yajnavalkya vomited the entire  knowledge which he had imbibed from the guru. 
On such an occasion the guru  asked his other disciples to take the form of partridges (Taittiriya birds) and  consume the leavings. It is said that for this particular reason this Upanishad  has been called the Taittiriya Upanishad. Instead of being repulsive, the story  has been given a completely different implication. It was said that Yajnavalkya  was such a genius that he was not only able to produce the teachings of the  Guru but had also added his originality and insight into the knowledge  acquired. Witnessing such a situation the guru was so charmed that he had asked  his other disciples to acquire the knowledge produced by Yajnavalkya as much as  they could. 
The Taittiri-birds or the  partridges are small in size but they are very active and careful and the  purport of the story is to instill into the students the spirit of the small  birds so that the disciples are as careful, as vigilant and as active as the  Taittri birds in the quest for knowledge. 
The essential content of all the  Upanishads is the same. However, in this Upanishad, there is no reference to  any war, any sorrow or any distress. There is not even the usual  student-teacher dialogue.  What we have  here is a spontaneous and exuberant expression of the essential content of the  original Vedic knowledge, as it is, presented in the freshness of an Upanishad,  with no reference to any particular situation in life. For most of us, this  Upanishad is as close to the pure Vedic education as we will ever have in our  lives. Ordinarily, Upanishads form part of the Jnana kanda of the Veda, which  deals primarily with Brahman, whereas the preceding Karma kanda of the Veda  deals with disciplines, rituals, meditations, values., etc. Because this  Upanishad is an overview of the entire Veda, it includes the essential content  of both the Karma and Jnana kandas of the Veda in that sequence.
Thus the first chapter of the  Upanishad called siksha valli gives  the essence of the  karma kanda of the  Veda in terms of disciplines, rituals, meditations, values, code of conduct for  daily life, etc. as Veda Upanishad , as moksha  sadhanam - as means helpful for gaining the overriding purpose of life,  namely moksha - total fulfillment in  life. The second and the third chapters of the Upanishad, called brahmananda valli and bhrigu valli respectively, give the  essence of the Jnana kanda of the Veda, in terms of knowledge of jiva, jagat and jagadeesvara followed by the step-by-step process of contemplation  on Brahman  leading ultimately to moksha.
Since this Upanishad deals with  the entire Vedic education, it is considered to be a complete Upanishad,  complete in the sense that it indicates all the necessary tools available to  every person for one's continued spiritual progress towards gaining total  fulfillment in life. Probably for this reason, this Upanishad is the most often  recited one on all auspicious occasions, bringing the benefit of Vedic  knowledge within the reach of ordinary people in daily life.