"Upanishad" is derived from the Sanskrit roots, "upa" or "nearby", "ni" or "at the proper place" and "sad" or "sitting". The word, then, strongly implies learning at the feet of a teacher.
After the Vedas, the Upanishads are the most influential scriptures in Sanatan Dharma. Essentially, they are the final part of the Vedas and are collectively know as "Vedanta" or "the end of Vedas". The exact number of Upanishads is not known (scholarly debate in the last 200 years suggests at least 250) but eleven are considered the oldest, most important and most authoritative. These are:
The Upanishads detail the characteristics of Parmaatma and they speak of an individual spirit, the aatman. Here, Parmaatma is often referred to as Brahman, which is the sum total of all existence past, present and future.