An agent is a standalone piece of software in Bongo, similar to a Unix daemon. Agents in Bongo tend to be very specialised and light-weight compared to their Unix counterparts: for example, a Unix MTA may speak SMTP, run a mail queue and deliver mail. In Bongo, that is separated into three separate agents.
Bongo's agents
Bongo comes with several agents. These include:
among others.
Writing your own
Check out the 'generic' agent (in src/agents/generic) for some information as to how you should write your agent.