More commonly known as pterodactyls, pterosaurs were winged reptiles - the first vertebrates to evolve powered flight. The evidence for flight comes from their light hollow bones, large brains and an extremely long fourth digit providing wing support. The discovery of large numbers of fossil species indicates that pterosaurs were initially highly successful.
Species ranged from the size of sparrows to the largest known flying creature of all time with a 12 meters wingspan. Towards the end of their reign, only the larger species remained as the smaller species were out-competed by early birds. Birds evolved flight separately to pterosaurs in a classic example of convergent evolution.
Pteranodon was almost certainly a soaring animal; it used rising warm air to maintain altitude; a common strategy among large winged animals (among birds, albatrosses and vultures are adept at soaring). Its scoop-like beak was used for snapping up fish as it soared over the oceans that it nested by. A good modern analog for Pteranodon would be the pelican.