Little Sisters are genetically enhanced female children between the ages of about 4 and 7 who roam the halls of Rapture looking for ADAM. When they come across a body containing ADAM (an "Angel") they stop and using their syringe needles they extract the ADAM from the body. They then move on to the next body, sipping the blood/ADAM mix in their syringe needles. Once the blood/ADAM mix enters their bodies the ADAM is separated by the ADAM slugs living like parasites inside of them.
Now should you get your hands on a Little Sister (that is if you can get past their protectors the Big Daddies whom they refer to as Mr. Bubbles or Mr. B), you have a few options. You can save the little sister by killing the slug in her. This will turn her into a normal little girl, you will get whatever ADAM she has on her and she will be forever in your debt which can come to an advantage later. The second option is you can harvest her which will result in a maximum ADAM gain. This can lead to more plasmids and will make it easier for you to survive. She will not survive the process however.
Your final option can only be achieved if you are also a Big Daddy. You can adopt her and act as her protector. Bring her to the "Angels" and keep her safe while she gathers ADAM. She will then give you a portion of her ADAM and after two gathers you can choose to save or harvest.
Jack approached the crying Little Sister slowly, Tenenbaum's and Atlas' voices echoing in his head.