We use the passive when the person doing the action is not important in the sentence. So an active sentence: Jo feeds the cat turns into a passive sentence: The cat is fed (by Jo).
The passive is also used in formal or factual writing or simply for style!
Passive Present: Use the verb "to be" with the past participle or 3rd form of the verb. For example: The ball is lost.
Passive Past Simple: Use the past of the verb "to be" with the past participle or 3rd form of the verb. For example: The ball was lost.
Passive Present Perfect: The ball has been lost.
Past Continuous: The ball was being lost.