They’re the last teeth to erupt in the back of your mouth. Usually, they erupt between the ages of 17 and 25. Occasionally, though, they find their way our much later than that; some never erupt at all.
Thanks to evolution, we’re evolving into the proud ownership of smaller jaws; unfortunately our teeth aren’t quite keeping pace. Most of our jaws only have room for 28 teeth; we have 32.
Basically, this means that the last teeth to erupt, which are the wisdom teeth, have nowhere to go if there’s not enough room remaining.