JOEL - The overall purpose for being sealed in the temple is so that a husband and wife and their children can continue their family relationship into the next life and so they can have the blessings of the gospel and priesthood more strongly realized in this life.
Most civil marriage ceremonies are only valid "until death do us part". Couples married and sealed in the temple are assured a continuation of their marriage into the eternities if they remain true and faithful to each other and to God.
The sealing ordinance must be performed by one holding the sealing power. The Savior referred to this sealing power when he gave his apostle Peter the keys of the kingdom of heaven, saying that "whatsoever thou shalt bind on earth shall be bound in heaven" (Matt. 16:19).
In modern times this sealing authority was restored to the earth in the Kirtland Temple on April 3, 1836, by the prophet Elijah, who was the ancient custodian of this power (D&C 110:13-16). Elijah appeared to Joseph Smith and Oliver Cowdery in the temple and transfered this power and authority to them so temple ordinance work could be performed.
Temple sealings are not only available to living persons, but are extended also to the deceased progenitors of a family through proxy ordinances performed in the temples. This process is known as salvation of the dead. It is one of the main reasons why we are so interested in genealogy.
Children born to parents who have been sealed in the temple are said to be "born in the covenant" and thus are bonded to their parents for eternity without a separate ordinance of sealing. Parents who adopt children would need to be sealed to them in the temple ordinance. Couples who have already been married civilly would also need to have the sealing ordinance performed.
A sealing ceremony is an inspiring and solemn ordinance performed in specially designated and dedicated rooms of a temple. The couple to be married or the family to be sealed kneel at an altar. The officiator is one who has received the sealing power under the highest priesthood authority in the Church.
Those who are married in the temple make a solomn three-way contract between each other and to God that they will be true and faithful to each other and to God. They must also be faithful to other promises that they have made in other odrinances in the temple in order for their sealing to remain in effect in this life and the next.
Marriages such as these can enjoy a happy and peacefull life, and can draw on a heavenly strength that can help them get through the tough times.