Confession is the popular term for the Christian sacrament more formally called Penance or Reconciliation. It is one of the seven official sacraments recognized in the Roman Catholic, Eastern Orthodox, and Old Catholic churches. Among Protestants, it is not considered a sacrament but is practiced in one fashion or another, often as a general confession recited by the entire congregation prior to receiving Communion. The admission of guilt usually is coupled with a request for forgiveness from sin.
One may confess privately to God, but the Bible calls on believers to confess to each other, and many denominations--especially the ones named above--expect member to confess their sins to a priest so that he can absolve the penitent of his sins or reassure him of God's forgiveness. In the Catholic Church, the penitent is customarily given a penance (certain prayers) to be performed so that there will be a sense of reparation for sin and to remove some of the temporal punishment consequent to sin.
In the Catholic and Orthodox churches, all mortal sins are to be confessed in order to obtain full forgiveness, while not all venial (lesser) sins need be expressed.
Sacramental Confession is usually made in a confidential manner, and clergymen are duty-bound never to disclose a confession. Numerous legal protections also respect this confidentiality in the very rare instance of an attempt to force disclosure.
Confession in a secular sense is any admission of a wrongdoing, written or spoken. In Church history, Confession in a non-sacramental sense has played a central role in Christianity from its earliest days, as in the example of the Confessions of St. Augustine.