The Upanishads form the core of Indian philosophy. They are an amazing collection of writings from original oral transmissions, which have been aptly described by Shri Aurobindo as "the supreme work of the Indian mind". It is here that we find all the fundamental teachings that are central to Hinduism the concepts of 'karma' (action), 'samsara' (reincarnation), 'moksha' (nirvana), the 'atman' (soul), and the 'Brahman' (Absolute Almighty). They also set forth the prime Vedic doctrines of self-realization, yoga and meditation. The Upanishads are summits of thought on mankind and the universe, designed to push human ideas to their very limit and beyond. They give us both spiritual vision and philosophical argument, and it is by a strictly personal effort that one can reach the truth.

Source: About.com

They deal with spiritualism and materialism.

Upanishads r like scriptures written by ancient yogis/rishi or saints...........
they can be extremely full of knowledge n wisdom or even tricks regarding tantra/ yoga/ ayurveda/ normal day to day life affairs etc
they can b contradictory or even false also at some occassions....... but most of them r full of ancient wisdom which can b still utilized for betterment of human life.

Scriptures and deal with knowledge and learning

They All Deal with the GOD and God's Creations !!: ))