Changelings are extremely well known in Western European folklore, and are quickly becoming more infamous in the North American cultures as well, inspiring creepy shows and movies all around. After all, there is very little more unsettling than a creepy child.
A changeling is typically described as being the child of a fairy, troll, elf or other legendary creature that has been secretly switched with a human child, though sometimes the term is also used for the human child who was taken. The 'child' that is left behind could also be an enchanted piece of wood that would soon appear to grow sick and die. The idea of a child being swapped out was often used as an explanation for children that mysteriously became sick, or developed mental or physical disorders in a family with no history of it back in the medieval years.
There were many reasons why a human child would be taken: to be used as a servant for those that took it, because the creature that stole it had fallen in love with the child, or because the creature holds a vicious grudge against the family. Fairies were most often to blame, though Norwegian tales accuse trolls of stealing them to prevent inbreeding, using the child once they've grown old enough to work new blood into the clans, and humans were given children with enormous strength as a reward. Some cases even tell of how older fairies, on the verge of death, would switch places with the child themselves so that they could live out the last of their days being pampered and coddled by their new 'parents'. Simple charms, like an inverted coat or open iron scissors left where the child sleeps, were thought to ward them off, though the idea of leaving open scissors around a baby seems just as dangerous... other measures included a constant watch over the child, though such a feat was difficult in older times, when farm labour was time consuming and required as much help from all family members as possible.
Another belief was that, again, trolls were behind it, though would only take unbaptized children, since they did not have the protection from Heaven yet, and since troll believed being raised by humans was classy, they were always eager to snap up any child left unblessed, which was why children were baptized as soon as possible.
In other folklore around the world, the changelings are switched with the child to feed off of the mother. The kidnapped child then becomes food for the changeling's mother. Other sources say that human milk is needed for fairy children to survive. In these cases, the newborn human child would be switched with a fairy babe to be suckled by the human mother, or the human mother would be taken back to the fairy world to breastfeed the fairy babies, though the theft of a mother was always thought to be very rare, and only attempted when the mother had lost a child, thinking that the chance to nurse a fey child in place of her own would comfort her.
Some changelings might forget they are not human and live out an entirely normal human life. Changelings who don't forget, however, may later return to their fairy family, often leaving the human family without any warning whatsoever, giving rise to the belief of being 'spirited away' by fey. As for the human child that was taken, they would often stay with the fairy family forever, not being given a choice in the matter.
This is only a brief summation of changelings; the lore on them is extensive, and changing for every country. The one constant, however, is that they steal children to leave their own in place, and it's next to impossible to prevent.
And as always, if any of you have tales or experiences of your own involving any of these beasties, please post said stories on the monsters' respective pages!!