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 up to 4 non-item granted perks (Needs confirmation).
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. At present, this is the extent of their involvement in the game.