|Skull of Triceratops|
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 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).