There are many divisions of dinosaurs, each division helps us understand the different species a little better. The first main division of dinosaurs is based on the shape of the pelvis. Saurischians are lizard-hipped dinosaurs while ornithischians are bird-hipped dinosaurs. However, it is important to know that birds, modern day dinosaurs, are not actually ornithischians. The division is based on a common ancestor that saurischians share and a common ancestor that ornithischians share. This means that although the ancestor of the saurischians had a lizard-shaped pelvis, not all saurischian dinosaurs had that shape. Some dinosaurs evolved to have different shaped hips than their ancestors.
Within both main groups, there are smaller divisions of groups of dinosaurs.
The saurischians are divided up into two groups: the sauropodomorphs and the theropods. Sauropods were a later group of sauropodomorphs and are the largest animals to ever walk the earth. They walked on all four legs and had very long necks. They were herbivores. Theropods only walked on their hind legs and came in many sizes. Most of them were carnivores, although some were herbivores. A tyrannosaurus-rex is an example of a theropod. A bird is also a kind of a theropod.
Ornithischians are divided into five main lineages: ornithopods, ceratopsians, pachycephalosaurs, stagosaurs, and ankylosaurs. These were all primarily herbivores, which explains the extra space in their ribcages. They used that space to help digest the plant material that they consumed because plants are much harder to digest than meat.
So those are the main groups of dinosaurs. Now go out there and sound smart!