There are three kinds of service opportunities for senior individuals or couples:
FULL-TIME SENIOR MISSIONARIES
These missionaries live away from home and are temple worthy, called by a prophet, and set apart by their stake president. These missions typically last 6 to 23 months.
CHURCH-SERVICE MISSIONARIES
These missionaries live at home and serve 8 to 40 hours per week in a work-related assignment for a Church department. Missionaries are temple worthy, called by their stake president and set apart by their bishop. These missions typically last between 6 and 24 months.
LONG-TERM VOLUNTEER
Long-term volunteers are not missionaries and are not called, set apart, or required to be a Church member. Volunteers typically live at home and serve fewer that 8 hours per week for 6 to 24 months.