The passive voice is a verb form that is used to describe actions or conditions. It changes the perspective of an action or event. While the active sentence emphasizes the subject carrying out an action, a passive construction places emphasis on the action carried out. The agent in a passive sentence is less important, perhaps not even named. The action itself is in the foreground. The subject of a passive sentence is usually the person or thing to which the process is done or happens.

Example:
Harry wird operiert.
(= In this sentence, what is happening to Harry - an operation - is of essence. It is not important who is actually carrying out the operation.)

Forming the passive voice
To form the passive, German uses the helping verb "werden" and a past participle. The helping verb is conjugated. The past participle remains unchanged. In a simple declarative sentence, the helping verb is in the second position and the past participle goes to the end of the sentence.

Examples:
Sie werden gleich operiert.
Hilfe! Ich werde getötet!
Dr. Anderson wird ins Krankenzimmer gerufen.

To change an active sentence into a passive one, the object moves to the beginning of the sentence or follows the finite verb.

Including the agent in a passive sentence
If you want to include the actor, cause or agent in a passive construction, that can be done in conjunction with certain prepositions.

"von" + an actor (dative):
Harry wird von Dr. Anderson operiert.
Harrys Haare werden von der Krankenschwester geschnitten.

"durch" + a cause (accusative):
Harry wird durch die Operation getötet.

"mit" + an agent (dative):
Harry wird mit einer Spritze beruhigt.

More:

Most accusative verbs can be used in the passive voice. The accusative noun or pronoun in an active sentence becomes a nominative noun in the passive sentence and determines the number and person of the verb. In a passive sentence it is the subject.

By contrast, the objects of dative verbs keep their dative form in a passive sentence. The word order is the same as for accusative verbs. Passive sentence constructions like these have no subject.