Passive Voice

Simply put, passive voice is a verb phrase; that is, a verb string consisting of a being verb plus a past participle; for example: The submarine was (being verb) sunk (past participle).

Also, writers often follow the verb with a prepositional phrase that begins with "by." The object of the preposition, the doer of the verb's action, should be the subject; for example: The submarine was sunk by the Germans.

Sometimes passive voice is necessary.

--from Joy Bagley, writer and workshop leader