An herb grinder (or simply, a grinder) is a cylindrical device with two halves (top and bottom) that separate and have sharp teeth or pegs aligned in such a way that when both halves are turned, material inside is shredded.
Here you can find all kinds of grinders. Plastic grinder, metal grinder, pocket grinders, 3 parts grinder, 4 parts grinder, painted grinder, themed grinder...