Adi Shankaracharya was the most prominent teacher of the Advaita school of Vedanta. His commentaries to the Srimad Bhagwat Gita, the Upanishads, and the Brahma Sutras define the parameters of Advaita thought. Although it must be kept in mind that all the Vedanta philosophies go back to the Upanishads.
Adi Shankaracharya is revered as the pre-eminent preacher who continued the Advaita tradition and "Shankaracharya" has become the title for the heads of the many Advaita schools in India today. This is because of the great fame and respect associated with it.
The philosophy of Advaita literally means 'non-dualism'. It is the oldest extant among the Vedanta schools of Indian philosophy. The unending quest, first begun in the Upanishads, is to decipher "brahma", the eternal source of the universe; the Atma (higher-self) and the Self (ego), and the mutual relationship between Atma and Brahma.