By-Agent - Passive voice
In a sentence with active voice, the subject of the sentence performs the action. This is expressed with the verb.
In the most Passive sentences the agent is not mentioned. If it is mentioned, it is usually done with the preposition "by" (by-agent).
English Passive 'by' (by-Agent)
Active
Tom writes a letter.
The classes use computers.
Passive
The letter is written by Tom.
Computers are used by the classes.