Before understanding Upanishads, it is important to understand their origin.
Upanishad means- to find spiritual knowledge and sit in meditation for God realization.
Upa- near, Ni- down, Sad- sit– i.e. Sit down near teacher for God realization
Vedas are the first form of knowledge, the primary scriptures. This is the most ancient source of any knowledge. Vedas are Shrutis-which means, which have been heard. After these were revealed to the Rishis while they were in a deep state of meditation, the knowledge was passed from one generation to the next by oral recitation and the process of oral transmission in called Sravana.
Upanishads contain spiritual and philosophical discussions about God, soul, and God-realization. These are link between Shruti and Shastras. It is explained by means of stories in Upanishads that one who can meditate upon God and surrender to God after having a full understanding of the real nature of the supreme being can be free from various sorrows and can attain “Moksha” or salvation.
There are considered to be about 108 upanishads but Eleven are considered to be the most authentic. The period of creation of upanishads is considered to be before the Ramayan and obviously Mahabharat era. There are several branches of Vedas and teachings of individual upanishads seem to have come from one or the other of these vedas. For example Brihadaranyak upanishad focuses on shukla yajurveda teachings and shvetashvatar upanishad on the other hand leans towards the krishna yajurveda. Names of the upanishad in many cases are based on the name of the rishi associated with that particular upanishad.
Some details about these 11 upanishads are as follows:
ISHA: This is basically the last chapter of Yajurveda with some minor variations. This is about Ishvara or creator. It is also called Vedanta.
Kena: is named so because it starts with word Kena-also called Talavakare Upanishad. This is about “who is ruled by whom”. This is supposed to be a part of Talavakara Brahamana. Belongs to Samveda School.
Katha: Deals with the inquisitive mind and soul. It is named after author.
Prashana: Inquisitive questions and answers by Sage Pippalada. Belongs to Atharvaveda School.
Mundaka: Belongs to Atharvaveda school-main teacher Angira. There are discussions about scientific and spiritual knowledge.
Mandukya: Discussions about consciousness and the super-consciousness. Also belongs to Atharvaveda School.
Aitareya: Named after author. Discussions about pure-consciousness.
Taittiriya: Qualitative discussions about soul and the realms of soul.
Chhandogya: Discussions about bliss and God realization. It is quite elaborate, highly illuminating and poetical.
10.Brahadaranyaka: This is last portion of Shatpatha Brahaman. Since the later is the liturgical exposition of Yajurveda and as such, the Brhadaranyaka Upanishad is a detailed exposition of the Isa Upanishad. Deals with the issues of self and creation. It is also quite elaborate
Svetasvatara: Deals with Reasons and descriptions of creation, soul, God, and the science of God-realization. Concept of the eternal entities, God, soul and nature is firmly explained.