Changelings
A Changeling is said to be a faerie that is exchanged for a human child, although the term can refer to the child who was taken. Usually (though not always) as a prank or an act of vengeance. Fairies are said to make this exchange if the human child's parents have caused the faerie world a serious offence, or if the fae have been attacked in some way by the parents. Rarely are children taken because the faerie is in love with it, though that is a possibility. Most modern stories indicate that the exchange is performed as a means of vengeance, and it is almost never depicted as a generous act.
On some occasions instead of a faerie child being left the faeries will leave a doll made of sticks and grass that is glamoured to look like a human child. These are called fetches and usually have a very short life span. Faeries will also sometimes take people who are older into their realm. Usually they do this if a specific quality about the person catches their eye.
Once in the faerie realm, humans are usually made servants. Since time passes differently in Arcadia, their faerie master determines how quickly they age. As children they usually just play while the faeries watch. When they are older they may be made a handservant, an entertainer, a lover, or even an ornament.
The magic of the faerie world changes the nature of the humans taken there so that, even if they do manage to escape, they are no longer fully human. The type of change that happens to them depends on who their master was and what they did while in the faerie realm. They almost always have a weak to intermediate grasp of faerie magic when they leave, with the ability to glamour and do other things.
Methods of supposedly repelling faeries included leaving an open pair of iron scissors on the baby's bed. The symptoms of a changeling includes unpleasant traits in the body, paleness, a green tint, bad temper, and/or a voracious appetite. "Positive" traits include an extensive vocabulary at a young age, which signified the changeling's intelligence. Children suspected to be changelings were persecuted and/or murdered, and those responsible were rarely blamed or punished.
Some speculate that the reasons faeries want to swap their children with human babies is because they want to be baptized, and consequently possess protection from things like trolls.
Read more about this topic: Classifications Of Fairies