The passive is a grammar construction that uses the auxiliary to be and the past participle of a verb:.
The passive has two main functions:
Firstly, we use the passive when we are more interested in what happened than who did it. For example, in saying My camera has been stolen the speaker is conveying important information about his camera. The camera is the focus of interest, and so the speaker has made it the subject of the sentence. He does not know or care who took it. Similarly, in the sentence The Mona Lisa was painted in 1503 the speaker wants to tells us when the painting was done. She is not interested in telling us who painted it, or maybe she expects us to already know that it was painted by Leonardo da Vinci.
The passive can be used in all tenses. The following list has examples of the most common uses:
Note: In all the above sentences, it is not important to the speaker that s/he tells us who (e.g., who cleans the classrooms, who is building the road behind the school, who saw the boy spraying paint). Important is: what (or when, why, how).
The second important reason why we use the passive is to follow the typical English sentence pattern of Given-New. This means putting given or old information at the beginning of the sentence (as the subject), and following it with new information (as the predicate). Example:
The second world war began in September 1939. It was caused by the invasion of Poland by German troops. At this time Poland was governed by the Polish Socialist Party.
Here is the alternative, putting the new information before the given or old, and using the active not passive voice:
The second world war began in September 1939. The invasion of Poland by German troops caused it. The Polish Socialist Party governed Poland at this time.
Most native speakers of English find this kind of text unusual and hard to read.
Note: You may read advice against using the passive. Microsoft's grammar checker in Word alerts you to uses of the passive. From this you may get the false idea that the passive is bad and should be avoided at all costs. If you want to read more about the passive, and when and how to use it, this UNC Writing Center webpage is an excellent starting point.