In the active voice, the subject of the sentence DOES the action:

Jake wrote a letter.
subject / verb / object

In the passive voice, the subject of the sentence is acted upon:

A letter was written (by Jake).
subject / verb

Notice that the object of the active sentence (letter) became the subject of the passive sentence. If we want, we can include “by Jake” to say who did the action.

This is a free sample lesson from the Advanced English Grammar Course.

Episode text - http://www.espressoenglish.net/passive-voice-in-english/

09 - Passive Voice in English

EspressoEnglish.Net