What us the difference between an ornithischian and saurischian?

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.