It seems like they have been around forever, but this generally seedless variety of Mandarin orange has only been common in the United States since the late 1990's, when according to Wikipedia, the harsh 1997 winter devastated the Florida citrus crop. Most clementines sold in the U.S. today come from Spain, Morocco or California. They have become immensely popular, at least in my household, due to their intense, sweet flavor, easy peelability and (usual) seedlessness. In other words they are easy and delicious, not a bad combination. Every once in a while though, we get a batch that are neither seedless nor delicious. Apparently the seeds come when they are cross-pollinated with other fruit by bees. Maybe that explains why the ones with seeds generally aren't as good, depending on what other fruits they are cross-pollinated with. While they are nice and juicy when fresh, I like them when they are somewhat desiccated too.
Clementines have become an essential winter citrus in my house, supplanting even the mighty, delicious and now expensive navel orange as our citrus staple, though I generally still prefer a good navel, cara-cara or blood orange to the clementine.