Follower
Followers are non-player characters (NPCs) who form an attachment to powerful characters and beings, as well as player characters, by agreeing to follow them as friends. Followers usually receive a share of whatever treasure is gained, but they don't work for a wage; this makes them distinct from hirelings. Followers are sometimes misconstrued as retainers, who are loyal servants attracted to levelled persons who obtain name level.
Gaining Followers
Followers can be collected throughout the campaign. If the players are honest, forthright and have shown a willingness to share and sacrifice themselves for an NPC, that NPC may choose (in the DM's opinion) to become a follower. Sometimes, a character may begin the game with childhood friends as determined by the character's background; or may acquire loyal family servants who no longer require pay. Followers may be met on the road, agreeing to throw in with the players for safety's sake while travelling. Other followers might be NPCs whom the player characters rescue, or who relate to the characters and grow attached to them. A monster might be a follower; even humanoids who would normally be thought of as traditional enemies might form an association with the player characters. Followers may be steadfast for life, or they may drift in and out of the players' lives. The trustworthiness and morale of a follower towards the party depends on how he or she is treated. There's no limit on how many followers the players has; but with too many followers, it's hard to give all of them the attention needed.
Relationships
Followers separated from the party will worry over the party's welfare; if the party is lost, they may raise a search party or seek some other means of communication. Followers can appear in the nick of time, saving the party from a massacre. Followers may be persons of reputation and means, who can say a word on the party's behalf or proffer a loan. One time followers may well rise to a position of authority, the benefits of which they'll share with the party they knew in those good old days, when adventure was the only thing that mattered. It is good to have friends.
A spurned follower, one who is mistreated, whose counsel is mocked or whose priorities are trampled upon, can become a deadly enemy. An ex-follower is far more dangerous than some random NPC the party once slighted. An ex-follower was invested once; and now they're hurt, misused and bitter. If there is anyone who might want to hire an assassin to kill a member of the party, it's almost certainly an ex-follower that the party felt it was okay to betray.
Running Followers
Obviously, the DM should not frivolously assign followers to the party, or make too much of some small rebuff on a player's part. Additionally, DMs must not use a follower to "run the party." The greatest contribution a follower should offer might be a tiny addition to a plan already intended by the party. Followers should offer knowledge but a minimum of insight. Followers are not there to problem-solve for the party. They "follow." They do not lead.
See Also,
Player Characters
The Adventure