Advaita Vedanta...

The teachings of Sankara can be summed up as:
“Brahma Satyam Jagan Mithya Jivo Brahmaiva Na Aparah—
Brahman (the Absolute) is alone real; this world is unreal; and the Jiva or the individual soul is non-different from Brahman.”

Tenets of Advaita Vedanta

• Brahman (the Absolute) is alone real; this world is unreal; and the Jiva or the individual soul is non-different from Brahman.
• The Atman is self-evident (Svatah-siddha). It is not established by extraneous proofs. It is not possible to deny the Atman, because It is the very essence of the one who denies It.
• Brahman is not an object, as It is Adrisya, beyond the reach of senses, mind or intellect. It is not another. It is all-full, infinite, changeless, self-existent, self-delight, self-knowledge and self-bliss. It is Svarupa, essence. It is the essence of the knower. It is the Seer (Drashta), Transcendent (Turiya) and Silent Witness (Sakshi). It is always the Witnessing Subject. It can never become an object as It is beyond the reach of the senses. Brahman is non-dual, one without a second. It has no other beside It.
• Sat-Chit-Ananda constitute the very essence or Svarupa of Brahman, and not just Its attributes.
• The world is not an illusion according to Sankara. The world is relatively real (Vyavaharika Satta), while Brahman is absolutely real (Paramarthika Satta). The unchanging Brahman appears as the changing world because of a superimposition of non-Self (objects) on Self (subject - Brahman). This is called Avidya.
• The Jiva or the individual soul is only relatively real. Its individuality lasts only so long as it is subject to unreal Upadhis or limiting conditions due to Avidya. The Jiva identifies itself with the body, mind and the senses, when it is deluded by Avidya or ignorance. Just as the bubble becomes one with the ocean when it bursts, so also the Jiva or the empirical self becomes one with Brahman when it gets knowledge of Brahman. When knowledge dawns in it through annihilation of Avidya, it is freed from its individuality and finitude and realizes its essential Satchidananda nature. It merges itself in the ocean of bliss. The river of life joins the ocean of existence. This is the Truth.
• Because samsara (or duality) exists due to ignorance or Avidya, Knowledge (Jnana) alone can make an individual realize his true nature. Karma Yoga, Bhakti Yoga, Raja Yoga etc., are necessary only to purify the individual and to help remove this Avidya. All other paths culminate in Jnana.
• Brahma Jnana is not about acquiring any external knowledge (as Brahman can't be an object of knowledge), it just about removing the Avidya or Maya.
  

Reference:
http://www.shankaracharya.org/
http://advaitaphilosophy.wordpress.com/2008/01/05/advaita-vedanta-philosophy-of-adi-shankaracharya/