Advaita is the non-dualism school of the Hindu philosophy of Vedanta, founded by Adishankarcharya.
Advaita teaches that the soul ([wiki]atman[/wiki]) and god ([wiki]brahman[/wiki]) are in reality identical and when one attains enlightenment the soul will dissolve into god like a droplet dissolving into an ocean. There is no such thing as an individual soul or even a world, all perceivied multiplicity and duality is an illusion ([wiki]maya[/wiki]) created by minds(jivas) only. Advaita is famous for the great sayings of Vedanta: Tat tvam asi, thou art that; ayam atma brahma, my soul is god; aham brahmasmi, I am god and prajnana brahma, god is consciousness.
It is the most popular Hindu school of philosophy and has been very influencial in shaping popular Hinduism. It also the most well known Hindu philosophy in the West and has had profound influence on shaping modern Western spirituality. Popular modern teachers of Advaita include new-age religious teachers Ekhart Tolle, Mooji, Gangaji.