The two cutting wheels inside a coffee grinder that fracture beans into a target particle size. They sit very close together, the gap calibrated to fractions of a millimeter, and the bean has to pass between them to come out the other side as ground coffee.
Two main shapes dominate specialty:
Conical burrs
A pointed inner cone sits inside a hollow ring. Beans drop in from above, gravity pulls them through the gap as the cone rotates, and grounds fall out the bottom. Lower RPM, lower heat generation, naturally bimodal distribution (a peak of fines plus a peak of medium particles). The Comandante hand grinder, the Niche Zero, and most espresso grinders use conicals.
In the cup, conicals tend to give cups with more body and a sweet bottom-end, sometimes at the cost of clarity.
Flat burrs
Two flat rings stacked horizontally. Beans drop into the middle, get thrown outward by centrifugal force as the bottom ring spins, and exit when they're small enough to escape the gap. Higher RPM, more heat, generally a tighter unimodal distribution. EK43, DF64, Mahlkönig X54.
In the cup, flats tend to give clarity and articulation — the cup that reveals what the bean is doing — sometimes at the cost of body.
Neither is "better." Conicals are forgiving and sweet; flats are precise and revealing. Many baristas keep both.