(1):
A noun suffix denoting an act; a state or quality; as in error, fervor, pallor, candor, etc.
(2):
A noun suffix denoting an agent or doer; as in auditor, one who hears; donor, one who gives; obligor, elevator. It is correlative to -ee. In general -or is appended to words of Latin, and -er to those of English, origin. See -er.