Although about 500 dinosaus species have been identified in the world, there are many more to discover. It has been estimated that about 1500 species have existed on the planet, but no-one has confirmed that there weren't more.
The three different types of dinosaurs are Theropods, Sauropods, and Ornithischia.
Sauropods were plant eaters, and they were the largest animals that have ever existed on the Earth. They had heavy elephantine bodies, small heads, long necks and long tails which often ended in a whip which they could use as a weapon. Their heavy weight meant almost all of them were walking on four legs, although their front feet were shorter showing their ancestors had been two-footed at some stage.
The third group, Ornithischia, included Ornithopods and armoured dinosaurs. All the members of the group were plant eaters, but they were much smaller than sauropods, and they had much shorter necks. But the main difference between the two were their hips (sauropods are "lizard-hipped" while Ornithischia are "bird-hipped".
While the carnivorous Theropods had three fingers, Ornithopods had five. They evolved relatively early in the dinosaur history, and despite being plant eaters (with large, compex digestive systems which made their bodies heavy), most of Ornithopods species walked on two legs. Ornithopods included Hypsilophodontidae, Iguanodontidae, Hadrosaurides, Lambeosaurinae, Hadrosaurinae, Edmontosaurini, Maiasaurini, Saurolophini, and Hadrosaurini.
Armoured dinosaurs belonged to the groups called Thyreophora and Marginocephalia. Thyreophora, such as stegosaurs and ankylosaurs, had large spines or plates on their back. Because these impressivly large weapons added to the weight of their body, these animals walked on four legs. Marginocephalia, such as ceratopsians, had horns on their heads, some quite impressive. Chasmosaurinis had two large horns attached to where mooses have their horns, while Centrosaurinae had one single horn on the nose, like today's rhinos.
NOTE: This website is written in British English, which is the English we use in Australia. You will find words like "traveller", "harbour" and "realise", and they are all correct in the language used in Australia.