The regular past participle is formed by dropping the er/ir/re endings and adding é for -er ending verbs, i for ir ending verbs and u for -re ending verbs.

aimer aimé		loved
choisir choisi		choosen
vendre vendu		sold

The above forms are regular, however there are a good number of irregular forms.

• The past participle is used to form the passé composé, which is used to express the past tense.

Guillaume a mangé tout le gâteau.
Guillaume ate the entire cake.

• It is used in passive sentences...

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

• it frequently serves as an adjective...

La porte est fermée.
The door is closed.

La boutique est ouverte.
The store is open.

Blessée, Marie boite maintenant quand elle marche.
Hurt, Mary limps now when she walks.

• and to form the present perfect.

Il avait dit la vérité.
He had said the truth.