Pachycephalosaurids are defined by their skulls: a thickened, often strongly domed roof of solid bone, in some genera many centimeters deep, sitting atop an otherwise fairly typical small-headed, bipedal body plan. What that dome was actually for has been debated for decades -- early interpretations favored head-to-head combat similar to modern bighorn sheep, while more recent biomechanical studies suggest flank-butting or purely visual display may have been just as, or more, important.
The family is known almost entirely from the Late Cretaceous, concentrated in North America and Asia. Pachycephalosaurus, the largest and best-known genus, carries the most pronounced dome of the family; Stegoceras, smaller and more completely known from multiple skeletons, has provided much of the anatomical detail behind the ongoing head-butting debate.