The obligation to verbally confess applies in a number of situations. Perhaps the best-known type of vidui is the one that is part of the process of teshuvah (repentance), when people confess and express their regret for a particular sin they have committed. Another vidui is recited by an individual offering a Torah-mandated sacrifice for a particular sin, or by an individual who is being subjected to punishment by a rabbinic court for a particular sin. Vidui is also relevant to sins in general. This includes the Kohen Gadol reciting a confession for the nation on Yom Kippur, an individual reciting the traditional confession on Yom Kippur, or a person on the brink of death reciting a final confession.
As part of teshuvah, a person must recite vidui for any transgression he or she has committed. This applies whether the transgression was of a positive or negative commandment, and whether the sin was performed willingly or unwillingly.
Essentially, the mitzvot of vidui and teshuvah are interconnected; for there is no vidui if there is no teshuvah. For if someone confesses his sins but does not resolve to avoid repeating them, he is like someone who immerses for purity while holding a dead (and thus impure) animal in his hand (tovel ve-sheretz be-yado). Vidui is necessary for the completion of teshuvah. Though a person who regrets his sins in his heart is deemed completely righteous, he still needs to confess verbally in order to finish doing the mitzva of teshuvah. First, he stops sinning, resolves not to sin again and stops thinking about it, and regrets having sinned. Then he says vidui, giving voice to what he has already thought. Nevertheless, if he is unable to verbalize the vidui, he should at least think it.
In Tanach, we find two types of vidui. One type is personal. Examples of this are the confessions of Kayin, King David, and Achan. The second type is collective. This can either be recited by an individual on behalf of the entire community (as did Moshe and Ezra the Scribe), or by the entire Jewish people collectively.
As we said above, our Sages stress that if someone has sinned and recites the vidui but continues to sin, he is like someone who immerses in a mikvah while holding an impure animal. It makes no difference how many bodies of water he immerses in – he is still impure. However, once he throws away the dead animal and immerses in a kosher mikvah, he is instantly purified.