Why turkey has the same name as Turkey
c.1400, agent noun from use (v.). Of narcotics, from 1935; of computers, from 1967. User-friendly (1977) is said in some sources to have been coined by software designer Harlan Crowder as early as 1972.
A person who uses narcotics, esp an addict (1950s+ Narcotics)
Users are looked down on by hackers to some extent because they don't understand the full ramifications of the system in all its glory. The term is relative: a skilled hacker may be a user with respect to some program he himself does not hack. A LISP hacker might be one who maintains LISP or one who uses LISP (but with the skill of a hacker). A LISP user is one who uses LISP, whether skillfully or not. Thus there is some overlap between the two terms; the subtle distinctions must be resolved by context.
The term "client" (as in "client-server" systems) is rather more specific, usually implying two processes communicating via some protocol.