Passive sentences emphasize the receiver of the action, by making what is normally the object of the verb the subject. Passive sentences are usually formed with the verb être and the past participle. The past participle will always agree with the subject. The perpetrator of the action need not be mentioned at all; if it is mentioned it is introduced by the preposition par and appears after the verb.

Malheureusement, Pierre a été mangé par des crocodiles.
Unfortunately, Pierre was eaten by crocodiles.

Le ciel a été illuminé par la foudre.
The sky was illuminated by the lightning.

La maison sera brûlée.
The house will be burned.

Elle est gravement blessée.
She is gravely injured.

In French, the passive is often avoided by using on, the "stand for just about anything" subject pronoun.

On vide les poubelles le lundi.
The trash cans are emptied on Mondays.

Dans l'usine, on fabrique des jouets.
In the factory, toys are made.

In addition, passive sentences are often formed with the pronominal. (See no external objects). Sometimes a pronominal has both passive and active interpretations:

La porte s'est fermée.
The door closed.

While othertimes it is clearly passive:

Les glaces se vendent mieux l'été que l'hiver.
Ice cream sells better during summer than winter.