A 'Confiteor' is a penitential prayer where we acknowledge our sinfulness and seek God's forgiveness. As a prayer form, it has been part of our tradition from nearly the beginning of Christianity. The one below was partially composed in the 8th century and then added to the Mass in the 11th Century.

Confiteor Deo omnipotenti, beatae Mariae semper Virgini, beato Michaeli Archangelo, beato Ioanni Baptistae, sanctis Apostolis Petro et Paulo, omnibus Sanctis, et vobis, fratres (et tibi pater), 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, omnes Sanctos, et vos, fratres (et te, pater), 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, to all the saints, and to you my brothers and sisters (and to thee, father) 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.

The current post Vatican II version of the Confiteor is a revision of the previous one.

Confiteor Deo omnipotenti, et vobis, fratres, quia peccavi nimis cogitatione, verbo et opere et omissione: mea culpa, mea culpa, mea maxima culpa. Ideo precor beatam Mariam semper Virginem, omnes Angelos et Sanctos, et vos, fratres, orare pro me ad Dominum Deum nostrum. Amen.

I confess to almighty God, and to you my brothers and sisters, that I have sinned through my own fault in my thoughts and in my words, in what I have done, and in what I have failed to do; and I ask blessed Mary, ever Virgin, and all the angels and saints, and you, my brothers and sisters, to pray for me to the Lord our God. Amen.