What are agents?

There is no widely accepted definition of the term agent that I know of. However, the (weak) definition of an agent given by Wooldridge & Jennings [Wooldridge 1995] is closest to my understanding of an agent (p. 116):

Definition: An agent is software program with the following properties:

• "autonomy: agents operate without the direct intervention of humans or others, and have some kind of control over their actions and internal state ...;
• social ability: agents interact with other agents (and possibly humans) via some kind of agent-communication language ...;
• reactivity: agents perceive their environment (...), and respond in a timely fashion to changes that occur in it;
• pro-activeness: agents do not simply act in response to their environment, they are able to exhibit goal-directed behaviour by taking the initiative."

But I am even more interested in several agents interacting with each other, which leads me to the following definition:

Definition: A multi-agent system (MAS) is a set of interacting agents.

If you want to know more about agent technology in general, please refer to the following books (or some other books): [Wooldridge 2002] [Weiss 1999]