Followers are NPCs who follow a Player/NPC with the leadership skill.
Only Civilians, Survivors, Big Bear and Famu Fataru can be recruited.
If a follower does not have a high amount of trust and becomes very hungry, they will leave you in search of food, and if hostile hungry civilians is turned on, may murder other civilians (possibly you?) if you are hoarding any.
Pressing the O key, you can give followers commands, whether to give you one of their items, to patrol a building, or to keep put while you scavenge for food. Ordering a follower requires that they have a certain amount of trust in you, which can either be generated over time using the Charisma skill, or gained through giving them items they feel they need or protecting them from zombies.