Viduy, confession, is an integral part of the Yom Kippur service; we say it each of the five times we pray the Shemoneh Esrei on Yom Kippur, both in the individual Shemoneh Esrei (at the end) and in the repetition of each Shemoneh Esrei (in the middle) by the chazzan. It is also recited in the individual Shemoneh Esrei in the Minchah before Yom Kippur, totaling ten times.
The confession of sins is an essential part of repentance. It is human nature for people to rationalize their own shortcomings in their own minds as unavoidable or even to define them as virtues. As long as someone refuses to acknowledge his wrongdoings, he cannot repent sincerely. So important is the confession, not only to the process of repentance but also to the Jew’s chance to survive the Heavenly judgment, that the Sages ordained that the confessional be recited the afternoon before Yom Kippur, lest illness or death prevent someone from praying on Yom Kippur itself. (The ArtScroll Siddur, p. 778)
Confession is accompanied by a sincere resolution to change. The Rambam writes in Hilchot Teshuvah (Laws of Repentance) 2:3:
“One who confesses with words but who has not committed in his heart to leave the sin is compared to someone who immerses in a ritual bath with an impure creature in his hand – his immersion does not help until he throws the creature away. Along these lines it is said, “One who acknowledges and leaves sin will be treated mercifully.”
Click here to see the fully translated text of the Viduy confession for Yom Kippur.