What us the difference between an ornithischian and saurischian?

Answer 1

Okay.

Ornothischian dinousaurs had bird-like hips - that is, both the ischium and pubis pointed backwards, like birds hips do. These dinosaurs were smaller herbivores.

Saurischian dinosaurs had lizard-like hips - the pubis pointed forwards, and the ischium pointed backwards. All theropods and some very large herbivores had these hips.

Ironically, birds evolved from the lizard-hipped dinosaurs, not the bird-hipped ones.

Answer 2

T.Rex is a theropod and therefore a saurischian ("lizard hipped") dinosaur.

Answer 3

T. rex is part of the theropods and is a saurischian aka "lizard hipped". This is due to the anatomy of it's pelvis bones. This also suggests something about its taxonomic ancestry to birds.