For the non-Mormon readers, I thought I'd post a small scale glossary of missionary terms that I'm likely to use.  This is certainly not exhaustive and is molded by my personal experience.  The reason I decided to post this now is that I have been cooking up a blog post for the last few days that draws on an experience from my mission.  For greater understanding of this and future posts, I thought I'd provide some definitions.

Missionary - Worthy and desirous Mormon men aged 19-25 and women 21-30 (? forgot the upper bound) can be full time missionaries.  Men's missions are for 24 months and women's for 18.  The title for a man is Elder and for a woman is Sister.  Most missions are proselytizing though humanitarian aid-only or other service-only missions exist too.  Missionaries are volunteer only and are not materially compensated for their time.  Typically, the missionary and his/her families pay for the missionary's support in the mission field.  There is also a church missionary fund that ensures all missionaries have funds for the necessities - housing, food, etc.

Mission / Mission field - this the place where a missionary works.  The mission name is a specific place but the term "mission field" is a general term.  For instance, I was a missionary in the Russia Moscow Mission, so for me Moscow, Russia and a few other nearby cities were the mission field.

President - each mission is headed by a mission president and his wife.  They are the priesthood authority for the missionaries and the top of the chain of command, so to speak.  The president's title is President and his wife's is Sister.

Assistant to President (AP) - typically, the mission president has two APs from the rank and file elders.  Their function is at the president's discretion.  Typical tasks for APs include - the top of the chain for gathering weekly statistics, training (missionary skills and language (for foreign missions)), and event organizing (conferences and other large scale meetings).

Zone Leader (ZL) - a mission is divided up into one or more zones.  Each zone has a zone leader who is similar in function to an AP but only for the scope of his zone.

District Leader (DL) - a zone is divided up into one or more districts.  Each district has a district leader who is similar in function to an AP but only for the scope of his district.

Area - a district is divided into one or more areas.  An area is a geographical location within a district where a single companionship of missionaries work.

Companionship - missionaries typically serve in pairs (though sometimes in threesomes).  A pair of elders or a pair of sisters are called a companionship.  Companionships are not mixed gender.

Trainer - when new missionary enters the field, his first companion is his trainer.  The trainer's responsibility, along with the standard duties he/she has in the area, is to train the new missionary in missionary skills and to help learn the language if it is a foreign mission.

Senior companion / Junior Companion / Co-companion - generally, the senior missionary (the one who has been in the field the longest) is designated as the senior companion while the other is the junior companion.  Senior companion is a title (like DL, ZL, or AP) and can be given to the companion with less time in the field.  If the two companions are not united in their opinion of what to do, where to go, etc., the senior makes the final call.  Some missions have co-companions or co-senior companions.  Here, there is no tie breaker vote, so to speak.

Stats - missionaries keep weekly stats on their activities.  These include metrics such as total proselytizing hours, non-proselytizing service hours, number of first time contacts, number of baptismal commitments, number of baptisms, etc.

Transfers - roughly once a month, new missionaries enter the field.  This is also the time when in the field missionaries might be assigned to new areas.  Transfers occur differently depending on the mission (more than anything how they happen is a function of the geography).  The one thing that is common is this - the assignments for companionships, areas, and leadership change during Transfers.

Office staff - the function of the mission office is dependent on the president.  Typically, the office takes care of things such as missionary apartments (when rent is due, the contracts, etc.), transfers transportation (getting to different cities or countries some missions span countries), cars (if applicable), referrals, membership records, etc.  The office staff take care of the day-to-day grunt work, so that the other missionaries can focus on proselytizing.  Office staff is often made up of two to four companionships of either elders or senior couples (that is, a husband and wife couple).