A very misused term.  Today, an agent seems to mean a stand-alone
piece of AI-ish software that scours across the internet doing
something "intelligent."  Russell and Norvig define it as "anything
that can can be viewed a perceiving its environment through sensors
and acting upon that environment through effectors."  Several papers
I've read treat it as 'any program that operates on behalf of a
human,' similar to its use in the phrase 'travel agent'.  Marvin
Minsky has yet another definition in the book "Society of Mind."
Minsky's hypothesis is that a large number of seemingly-mindless
agents can work together in a society to create an intelligent society
of mind.  Minsky theorizes that not only will this be the basis of
computer intelligence, but it is also an explaination of how human
intelligence works.  Andrew Moore at Carnegie Mellon University once
remarked that "The only proper use of the word 'agent' is when
preceded by the words 'travel', 'secret', or 'double'.