Missionary work is one of the most important aspects of LDS life. A mission is an expedition to spread the gospel to new people, aimed at expanding the church's membership. Missionaries spend two years at their assigned missions, often in foreign countries, sharing information about their beliefs and inviting people to convert to Mormonism. The church's official website explains the importance of missions thus: “Imagine you found a cure for cancer. How urgently would you spread the news of your discovery? Who would you tell? The gospel of Jesus Christ is the cure for so many of life's ills that Mormons want to share the good news of eternal life with the same urgency."

More than 80% of young men who are active in the church go on a two-year proselytizing mission, usually starting at age 19. Prospective missionaries must meet certain “standards of worthiness" (including regular church attendance, prayer and adherence to the Law of Chastity and Word of Wisdom), which are confirmed by their bishop before applying.

Once approved, each missionary receives a “call to serve," assigning him to one of the 405 LDS missions located throughout the world. Male missionaries must also be ordained to the Melchizedek priesthood (held by all “worthy" adult male Mormons), which grants them the authority to lead meetings, to bless and minister to the sick, and to ordain others to the priesthood. The lowest of the five offices within the priesthood is called an Elder; male missionaries go by the title “Elder" and their last name, and after the mission they may continue to rise through the ranks.

Although women are not allowed to hold the priesthood, they can serve as missionaries; about 13% of LDS missionaries are young women. All missionaries spend several weeks at a Missionary Training Center before they depart, and those who need to learn a new language for their mission are encouraged to study by total immersion in order to become fluent as quickly as possible.

Missionaries have a very strict lifestyle, following a daily schedule of prayer, scripture study, and proselytizing in the community. They are probably best known for going door to door, visiting “investigators" (potential converts) at home, but the job also includes community service, and most missionaries will try to be helpful to the people they meet—to do good in their new communities as part of practicing what they preach. They are assigned to each other in pairs, called companionships, which are always of the same gender. Companions not only live together for the duration of the mission but must accompany each other at all times, except inside the bathroom. For the duration of the mission, they are forbidden from being alone with anyone of the opposite sex; they are also required to avoid “worldly entertainment" and media (except to email their families), non-religious music, and using slang words.

Some quotes about missions from the LDS official website:

What will the Mormon missionaries talk about when they visit my home?

Missionaries share a message about Jesus Christ and His Atonement for all people. They teach about our Heavenly Father's great plan of salvation, which allows all people the opportunity to return to Him.

Katie answered...

"As a missionary, my purpose is to invite others to come unto Christ. I and other missionaries like me do this by teaching about Christ and His Atonement. We also teach people about God's plan for us--how we can return to Him and become more like Christ. We help people identify, make, and keep the commitments that will lead them to Christ."

Josh answered...

"Missionaries "invite people to come unto Christ by helping them to receive the Restored Gospel through faith in Jesus Christ and His Atonement, repentance, baptism, receiving the Gift of the Holy Ghost and enduring to the end." Missionaries throughout the world memorize this "purpose statement" and try to let it guide everything they do. The missionaries will teach you about God, your relationship with Him, His Plan of Salvation (through which we may return to live with Him and with our families), the Gospel of Jesus Christ and about His Atonement, and God's commandments. They will also invite you to find out for yourself if the message is true. Inviting you to act is one of the most important things that missionaries will do. Only through your own efforts can you receive a witness that what the missionaries teach you is true."

Marco answered...

"When missionaries of the Church of Jesus Christ of Latter day Saints visit your home, they will share with you the truths of the Plan of Salvation. You will come to know about the purpose of life and the way we can return to our Heavenly Father and live forever in a state of Joy with those we love. You can ask them questions that will help you to better understand how to live the Gospel of Jesus Christ in your life and receive all the blessings that came from the obedience to God's laws. They will teach you about the Restoration of the Gospel and of the Church of Jesus Christ. They will bring a sweet Spirit of joy and love to you and your family as you prayerfully listen to them. They will tell you why we believe that families are forever. I had the privilege to serve as a full-time missionary in the Paris France Mission; those two years were the best years of my life; I met many people that become eternal friends with whom I shared experiences that have made me a better man, husband, father, citizen and worker. I promise you will have a great experience as you invite these young Servants of the Lord into your homes!"

From an NPR interview with Ryan McIlvain, an ex-Mormon author:
On how his mission to Brazil became a way of convincing himself of his own faith

"I was a doubter almost as soon as I became aware of what I believed. I became aware at maybe 13, 14 that I'd been drafted onto this team called Mormonism that was distinct from the Catholicism that was the norm where I grew up in Massachusetts. And I started to become aware of those differences, and instantly I was doubting them, at first privately and then, by the time I was in my late teens, quite vocally. But some of the people who were very close to me — my parents, certain friends in my congregation, or ward, as it's called — their example meant a lot to me, and I thought, 'What do they know that I don't know?' So the mission for me became a chance for me to make a large commitment, commensurate to the large amount of grace I felt I needed in order to believe what seemed, on the face of it, unbelievable things."