There are two types of coffee grinder, one of which uses blades and the other, two interlocking rough-edged or "burred" cones or a pair of plates to grind coffee beans. While those with blades are referred to as "grinders," they actually slice, shave and chop up the beans into small pieces.
Blade grinders, which are typically less expensive than burr grinders, produce grounds of inconsistent size, ranging from a fine powder to coarser pieces, with the smaller particles tending to clog espresso machines and French presses. An inconsistent grind also makes for less predictable flavor in the final product. Many coffee aficionados opt not to purchase blade grinders for both of these reasons.
Burr grinders actually grind the beans. With the plate versions, one plate remains immobile while the other turns, pulverizing beans between them. In the case of grinders using burred cones, an inner cone rotates, grinding beans against an outer cone, which remains stationary. These grinders produce grounds of uniform size and consistency, which permits the user to control the overall coarseness of the final product. Coffee drinkers using a drip brewer generally prefer a medium-coarse grind, while those using a French press typically choose a very coarse grind.