An agentive ending in the English language is the use of the suffix -er, -or, -ist, or -ian at the end of a verb in order to create a noun meaning "someone or something that does the action the verb describes." The corresponding suffixes are called agent suffixes.
Examples include provider (from provide) and builder (from build), pianist and librarian.
There is no systematically clear distinction between the ending chosen for a person who does the action and for a machine that enables the action to be performed: though there is a distinction between a typewriter (machine) and a typist (person), a cleaner can be a washing substance or a person who scrubs with it.