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.
Today's lesson is the complete guide to the passive voice in English. You'll learn when to use the passive voice, how to form the passive voice in all English verb tenses, and more.
This is a free sample lesson from the Advanced English Grammar Course.
Click here for the episode text - http://www.espressoenglish.net/passive-voice-in-english/