One that performs a specified action: accelerator.
State; quality; activity: valor.
Suffix appended to terms to create an agent noun, suggesting somebody who does some thing.
A noun suffix denoting an act; a state or quality; like in error, fervor, pallor, candor, etc.
A noun suffix denoting a realtor or doer; like in auditor, one who hears; donor, person who provides; obligor, elevator. It's correlative to -ee. Overall -or is appended to terms of Latin, and -er to those of English, source. See -er.