Any one who performs any action on a work item is an agent....and agent needn't be an approver.
If the action on work item requires any approval, agent becomes an approver.
Eg In performance mgmt workflow you submit ur objectives to your manager & he might add on few more obj & submit it for your review...so here you & your manager both become agents...
Rewards Points if this satisfies you
Cheers
~Arun
