The priesthood has been vested in chosen individuals since the world began. Bible prophets and apostles all held this power. Through it, they received guidance and truth from the Lord, performed miracles and blessings for the benefit of those whom they led, and officiated in important, sacred rituals and ordinances such as baptism. In general terms the priesthood serves as a special and official connection between heaven and earth, between God and man, between Christ and His Church.
There have been times in history when people of the earth have lost the priesthood through disobedience to God's commandments. They have rejected and fallen away from the truth, and by so doing have been left with a severely decreased understanding of the workings of God. Among Mormons this state is known as apostasy. The worst of these periods was called the Great Apostasy, occurring after those Jesus Christ left to govern His Church, the apostles, were murdered, along with many other faithful followers of Christ. The pure teachings of the Church were corrupted, and new religious organizations were established without divine sanction or authority. The western world was plunged into a time of spiritual and secular ignorance known commonly as the Dark Ages.
Each period of apostasy has been followed by a restoration of divine teachings and authority to the earth. Joseph Smith was chosen by God as the prophet who should restore the true Church of Jesus Christ and the priesthood in our time. So it was, that in 1829 he received a degree of the priesthood by the hands of the heavenly messenger, the resurrected John the Baptist. Later Joseph received the priesthood in completeness from Jesus’ apostles Peter, James, and John, also in their resurrected state. Thus, Jesus Christ passed His authority on to His apostles, and they later passed it to Joseph Smith, who conferred it upon others. They, in an unbroken chain, have passed it on to worthy male members of the Mormon Church up to this generation.
In this, the modern Church of Christ, the priesthood is structured in several different stages or offices. The first degree of authority is called the Aaronic Priesthood, after Aaron of the Old Testament. Men who hold this degree are in the office of a deacon, teacher, or priest, progressively. The completeness of divine authority is bestowed in the form of the Melchizedek Priesthood, and has offices including elder, high priest, seventy, and apostle. These various offices are in place so that the individual priesthood holder can find personal growth and development as he performs increasing duties and service for others in an organized manner.
One duty of the priesthood is the blessing of what Mormons call the sacrament, or sanctified bread and water that represent the body and blood of Christ, the regular partaking of which constitutes a special promise of mutual faithfulness between Jesus and his followers. Other duties include baptism by water, the giving of the gift of the Holy Ghost, yet other special ordinances or rituals performed in Mormon temples such as marriage for eternity, the collection of donations on behalf of the poor, and many other special acts of responsibility and service. Through the priesthood a man can bless his family members and others to overcome sickness and to receive increased peace, strength, and guidance from the Lord. Overall, the priesthood is a special gift from God designed to touch and guide the lives of all His children on earth, so long as there are individuals willing to receive His gift.