The school of Hindu philosophy most concerned to protect the literal truth of the Upanishads, and therefore most critical of the dualistic and realistic tendencies in other Indian philosophical schools. The purest monism (advaita or non-duality) recognizes only brahman (identified with atman, as the Upanishads require), and is especially associated with the philosopher Shankara.
http://www.vedanta.org/wiv/overview.html An overview of the Vedanta with links to other sites