The people in your fort are called survivors. Survivors exist to accomplish various missions, which serve to acquire resources for your fort, make it safer, or otherwise increase your fort's chances of continued existence. Survivors not assigned to a specific mission remain inside the fort on Guard Duty, keeping other survivors safe from marauding zombies.
Unless altered with a perk, each survivor needs a house to live in, and will consume 1 unit of food per day. Every survivor is rated in five different skills, has a happiness level from 0-100, can equip one weapon and one nonweapon, and can have various perks.
Some survivors may have a child with them, which function as a handicap for characters that have them "equipped". Children function as a non-weapon item that decreases all stats but boosts happiness. Unless the fort policy "Children must go to school" is in effect, each child must be equipped by a survivor or else the game will automatically assign a survivor to equip the child. Each child has a mouseover tooltip that shows their age, and children that are 13 years old do have a chance to "grow up" at 14 and be converted from a non-weapon item to a full-fledged survivor that needs housing and can be sent on missions.