A changeling is the offspring of a faerie that is left in place of a stolen human child. Some of the signs that you have a changeling instead of your own child would be if the child wasn’t growing up as it should. If a baby cries all the time or is very sickly, for example, it might be a changeling.
Reasons why faeries take human children away could range from desiring a human child for their own, to acquire a human slave, thinking that the child will have a better life living with the faeries, etc. To prevent this from happening, parents might put iron or steel above the door to the child’s room or to baptize the child as soon as possible.
This essay by D.L Ashliman is a wonderful resource that brushes over many topics. Legends, how changelings were treated in history, what they were thought to be, etc. http://www.pitt.edu/~dash/changeling.html And on the same page here are several changeling legends http://www.pitt.edu/~dash/britchange.html
Some books you can find changelings in are
Heir to Sevenwaters by Juliet Marillier
Tithe by Holly Black
The Moorchild by Eloise Mcgraw
The Stolen Child by Keith Donohue (also the poem by the same name is about a changeling)
Outside Over There by Maurice Sendak
Confessions of an Ugly Stepsister by Gregory Maguire
Just to name a few.
The story Rumplestilskin would also be an example of a situation where a faerie takes away a human child, but in that case he doesn’t technically leave a changeling behind. In some versions of the story he leaves behind a straw version of the child, which could be interpreted by some as the changeling, as faeries are in some cases composed of natural elements.