Paleontologists know more about Iguanodon than they do about most dinosaurs--partly because this was one of the first dinosaurs ever to be discovered (in England in 1822, by Gideon Mantell) and only the second ever to receive a name, after Megalosaurus.
For example, one of the things we know is that there was definitely more than one type of Iguanodon--though the exact classification is still a matter of some dispute. Fossils have been found as far afield as Asia, Europe and North America, and it's unclear exactly how many individual species ever existed, or how they were interrelated.
One thing we do know is that Iguanodon was a relatively gentle plant-eater, despite its enormous size (twice as heavy as an elephant) and the scary-looking, foot-long spikes on the ends of its thumbs. It's thought these spikes may have been a means of defense against carnivores, or perhaps just a handy appliance for breaking open tough-shelled fruits.
