An ELT Glossary : Agent
The agent of a verb is the person, thing etc which performs the action described by the verb. Thus in sentences containing active verbs, the agent will be the subject of the sentence :
Ann painted the kitchen yesterday.
In this sentence, Ann performed the action, so Ann is the agent. It's an active sentence, so Ann is also the subject of the verb - painted.
If the verb is passive, however, the agent is no longer the subject. The agent may not be mentioned at all ...
The kitchen was painted yesterday.
Or may be included in a by phrase :
The kitchen was painted by Ann.
Another way of avoiding mentioning the agent in a sentence is by using an ergative verb : The door opened. /The lesson started./ The water boiled.