Geluk (Tibetan, dge-lugs). The largest of the four main orders of Tibetan Buddhism, and that to which the Dalai Lamas belong. Founded by Tsongkhapa in the 14th century, the school became dominant in the 17th century under Dalai Lama V and has remained the most influential down to modern times. This tradition integrates the methodical study and practice of the sūtras and mantras developed by Tsongkhapa and his successors, based in part upon the older Kadampa lineage which derived from the teachings of Atiśa. In addition to an emphasis on textual study, the school also stresses the importance of compliance with the monastic code (Vinaya). The monks of the Geluk school have also been known traditionally as the ‘Yellow Hats’ because their ceremonial hats are yellow, as opposed to the red hats of other schools. See also Tibet.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)