A Confiteor is a Catholic penitential prayer where sinfulness is acknowledged and God's mercy and forgiveness are sought. Confiteors have been part of Christianity from the beginning. St. Augustine notes that it was traditionally recited while striking the breast as a sign of humility. The prayer below is the traditional form of the prayer. I've added it to this node, with the translation, because it is the official Church translation and clears up some of the ambiguities in the above. This one was partially composed in the 8th century and then added to the Mass in the 11th Century. The prayer is also called the Confiteor Deo and is sometimes used during the Sacrament of Extreme Unction.

Confiteor Deo omnipotenti, beatae Mariae semper Virgini, beato Michaeli Archangelo, beato Ioanni Baptistae, sanctis Apostolis Petro et Paulo, et omnibus Sanctis, quia peccavi nimis cogitatione, verbo et opere: mea culpa, mea culpa, mea maxima culpa. Ideo precor beatam Mariam semper Virginem, beatum Michaelem Archangelum, beatum Ioannem Baptistam, sanctos Apostolos Petrum et Paulum, et omnes Sanctos, orare pro me ad Dominum Deum nostrum. Amen.

I confess to almighty God, to blessed Mary ever Virgin, to blessed Michael the Archangel, to blessed John the Baptist, to the holy apostles Peter and Paul, and to all the saints that I have sinned exceedingly in thought, word, and deed, through my fault, through my fault, through my most grievous fault. Therefore, I beseech blessed Mary ever Virgin, blessed Michael the Archangel, blessed John the Baptist, the holy apostles Peter and Paul, and all the saints, to pray for me to the Lord our God. Amen.