"The happiest people I know are those who lose themselves in the service of others." - Gordon B. Hinckley
Missionaries for the Church of Jesus Christ of Latter-day Saints are often young men and women, ages 18-21 (though some are older!), who serve for 1.5-2 years to share the gospel of Jesus Christ. They put their lives on hold - school, jobs, and a social life - to live as Christ lived, selflessly serving others. Aside from teaching, a typical day for them also includes service work for neighbours or visiting seniors to brighten their day.
Prior to serving a mission, they often work a part or full time job back home to save up money they can use to cover basic expenses like food, rent, and personal items while they're out on their mission. They're not paid at all for their volunteer service during the 2 years and they are to live as frugally as possible, focusing their energy on serving others rather than themselves.
They also don't get to choose where to serve! They wait until their "mission call" letter that tells them where they will be serving, which they often open in front of their closest family and friends!
If you've met them in person, you may wonder why they're called "Sister" or "Elder" - it's just a title given to them during their mission and signifies their role in furthering the Lord's work! (I too, wondered this, when I first met them!). If you see them in your area, don't be afraid to say hi!