JOEL - When Christ was on the earth He confered priesthood authority when He ordained His apostles:

"Ye have not chosen me, but I have chosen you, and ordained you, that ye should go and bring forth fruit, and [that] your fruit should remain: that whatsoever ye shall ask of the Father in my name, he may give it you."(John 15:16)
"And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven." (Matthew 16:19)
"FOR every high priest taken from among men is ordained for men in things pertaining to God, that he may offer both gifts and sacrifices for sins:
And no man taketh this honour unto himself, but he that is called of God, as was Aaron"(Heb 5:1,4)

This "ordination" and "keys of the kingdom of heaven" refer to priesthood authority. With this authority the apostles of Jesus could perform ordinances that would not only be bound on earth but also be bound in Heaven. Our Church claims that after Jesus was crucified and the apostles all died off, this priestood authority was lost from the earth because of the apostasy from the faith as was predicted (Acts 20:29-30, 1 Tim. 4:1-3, 1 Tim 4:1-3, 2 Thes 2:3)
Because "no man taketh this honor unto himself" it was necessary for God to restore this priesthood authority as also predicted(Rev. 14:6-7). He did this by sending angels; His apostles, Peter, James, and John to earth in 1829 to Joseph Smith. They laid their hands upon his head and confered upon him the priesthood of God. With the proper priesthood authority now on the earth again, Joseph was able to restore the Church of Jesus Christ and once again we are able to perform the saving ordinances necessary for our salvation and eternal life.
Priesthood in our church is not a vocational or professional position. It is not hereditary, passed by inheritance from father to son. It is not obtained by money. It is not held by a group of specialists who are separated from the community. And it is not a "priesthood of all believers," as in the Protestant conception. In the LDS church priesthood authority is passed on from one priesthood holder to another by the laying on of hands during the ordination prayer. Every priesthood holder in the Church can trace his line of authority back to Joseph Smith; Peter James and John; and finally to Jesus Christ.

The word "priesthood" has several meanings for Latter-day Saints:
1.Priesthood is the power of God, a vital source of eternal strength and energy delegated to men to act in all things for the well-being of mankind, both in the world and out of it.
2.Priesthood is authority, the exclusive right to act in the name of God as his authorized agents and to perform ordinances for the purpose of opening certain spiritual blessings to all individuals.
3.Priesthood is the right and responsibility to preside within the organizational structure of the Church, but only in a manner consistent with the agency of others.
4.Sometimes the word priesthood is used to refer to the men of the Church in general.

All active LDS men and boys over the age of 12 are given different levels of priesthood authority. Boys ages 12 to 18 are given the Aaronic priesthood. They are ordained to one of three levels of office; Deacon, Teacher, or Priest. Normally they advance from one office to the next depending on their age. Their priesthood duties include the preparation and administration of the emblems of the Sacrament each Sunday. They visit other members of the congregation and render assistance where needed. A Priest can perform baptisms. When a young man becomes about 18 years old he is given the Melchezidek priesthood and ordained to the office of an Elder. Later on in life he may be ordained to the office of a High Priest. Those of the Melchezidek priesthood can do everything done by the Aaronic priesthood plus many additional responsibilities. They can bestow the gift of the Holy Ghost on a newly baptized member; they can administer to the sick and use priesthood power to heal them; they serve in leadership positions within the organization of the church.
The Bishop(Pastor) is the spiritual leader of a congregation of members on the local level. Among other things he is authorized to perform civil marriages for those members who reside in his jurisdiction. However, most members try to prepare themselves for an eternal marriage, which is performed in our temples by someone who has been given special priesthood authority to seal a couple for time and all eternity.

The following ordinances are the religious rites of the church performed by those with priesthood authority:
Baptism
Bestowal of the gift of the Holy Ghost
The Sacrament
Administration to the sick
Conferal of priesthood authority and ordination to a priesthood office
Temple Endowment
Temple Marriage and other sealing ordinances
Naming and blessing of children
Consecration of oil (used for administering to the sick)
Dedication of buildings and graves

The main function of the priesthood in the Church is one of service. A man who holds the priesthood of God is obligated to use it for the benefit of the Church and its members and for himself and his family.