Marginocephalia

Marginocephalians (meaning "fringed heads") are a group of Ornithischian (bird-hipped) dinosaurs. 
They can be briefly described as "herbivores which defend themselves with armoured heads". They are in two groups: the thick-skulled Pachycephalosauria and the horned Ceratopsia. They are all herbivores. 
The Pachcephalosaurs had very thick skulls which (it is supposed) they used for defence. They developed in the mid-Jurassic, and are common in Cretaceous rocks. 
The Ceratopsia had horns, and we know they used them on predators. "Triceratops" is the key example.
Some of their features are not so obvious: They have a unique palate (part of the mouth) and a short pubis (part of the hip).