Bioshock > Adam > Little Sisters and Big Daddies
A Little Sister (also called a Gatherer) is a little girl, in whose stomach lining is implanted the Adam-producing sea slug, turning her into an Adam factory. She is also psychologically conditioned to wander Rapture looking for dead aggressors; when she finds one, the Little Sister extracts the corpse's blood with a syringe-like tool and drinks the blood (noisily) to process the Adam therein. They are also trained to bond with the Big Daddies for protection, and to crawl through the air ducts for travel.
Due to Adam saturation in their bodies, Little Sisters are invulnerable to most injuries. (You can see them glow as they heal.) But they can die. You find evidence at the start of the Port Neptune level of a Little Sister crushed to death in a mass of rubble. (You don't see the body, but the syringe, the dead Protector and the blood are there.) And they die when the sea slug parasite is torn out of them.
To protect the Little Sisters from attack by aggressors desperate for Adam, Dr. Suchong developed the Big Daddies, or Protectors, by grafting a subject's skin, muscles and organs into one of the diving suits left over from the days of Rapture's construction. Then plasmids are injected to inhibit the Protector's intellect and will, and to make them protect the Little Sisters; and pheromones are applied to bound the Littles Sisters to them. (The proof that this bond works was the last act of Dr. Suchong's life, as a Big Daddy he was working on impaled him after he slapped a Little Sister in anger.)
Big Daddies wander the halls and rooms of Rapture until they reach an air duct. Then they bang on an air duct to summon a Little Sister. If one comes out, it follows her in her search for dead aggressors, and stands on guard while she does her work. Anyone coming too close to the Little Sister is pushed back by the Big Daddy. Most aggressors know enough to give it a wide berth. If one should attack, the Big Daddy springs into action, attacking the aggressor with remarkable speed and strength until the attacker is dead or (less likely) driven away.
There are two types of Big Daddy. Bouncers are stoutish hulks with many eyes; they come equipped with a conical mining drill and a mean ramming attack. Rosies are more normal looking; are armed with a rivet gun; and can also toss proximity grenades. Both come in regular and (starting in Fort Frolic) enhanced 'elite' versions.
The eyepieces on both glow yellow while on patrol, red during attack, or green when placid (as during repairs or when hit with a Call Big Daddy plasmid).
You can research the Little Sisters and both types of Big Daddy. Little Sister research gives you small increments to your health and Eve. Big Daddy research is more fruitful. More on this in the Research page.
The Little Sisters produce Adam. Aggressors are not the only ones who need it. So does you, as Jack, and what you do with a Little Sister after you kill her Protector is the moral fulcrum of the game.
It has been found that harvesting the Little Sisters pays off big at first, but the advantages taper off after a time.
On the other hand, rescuing the Little Sisters makes you an ally in Dr. Tenenbaum. After saving every third Little Sister, she gives you 200 units of Adam, special plasmids (the Call Big Daddy kind) and other goodies as teddy-bear gifts in front of Gatherers Garden machines. These gifts can be the difference between life and death, esp. in the more difficult later levels.
The choice, of course, is up to you.