The word ‘Upanishad’ mean ‘sitting devotedly near’. The Upanishads are revealed by God at the commencement of each cycle of creation to the worthy few Rishis. It is sitting at their feet that we have to learn them.
The Muktikopanisad gives a list of 108 Upanishads, out of which the principal Upanishads are
The minor Upanishads are sometimes grouped as follows:
a) Shaiva Upanishads
The Shaiva, the Sakta and the Vaisnava Upanishads deal with respective cults of Shiva, Devi and Visnu. The Yoga Upanishads supply a lot of information about Hathayoga and Rajayoga based on the Yogasutras of Patanjali and other works. The Samnyasa Upanishads deal exclusively with monasticism, its ideals and practices.
Various Commentaries on the Upanishads
Sankara is the earliest and the first to comment upon the Upanishads. His bhashyas or commentaries, further elucidated by the tikas or glosses of Anandagiri (13th century) are invaluable source books to unlock the wisdom of the Upanishads.
Ramanuja has not commented upon any of the Upanishads though he has tried to amplify some of their concepts in his Vedartha Sangraha. Rangaramanuja (circa A.D. 1600) has completed the work by commenting on all the major Upanishads Madhva (A.D. 1197-1276) has written brief commentaries on all the ten ancient Upanishads upon which some like Raghavendra Tirtha (A.D. 1595-1671) have written glosses.
A later writer, Upanishad Brahmayogi, has done remarkable work by commenting on most of the minor Upanishads.