Grinders are very useful if you want to cut up herbs or similar stuff. They can be used for kitchen herbs as well as smoking herbs. The most grinders are available as two-part mills with metal jags on one part. Just put your herbs in and turn the lid. The result will be surprisingly good.
Herb Grinder with sieve.
Advanced herb mills have an additional sieve in the middle of the two parts. That filter provides finer results and better substances to consume. Usually grinders with sieve are completely made of metal, but simpler mills could also contain wooden parts or are made completly of acrylic. These mills are effective too but not as good as the metal ones.
Too lazy to screw your grinder. The latest models have an electric system working with batteries or electricity cables. Just push a button and let your grinder do all the work. But, of course, if you are an old-fashioned nature, you can also use the good ol' ceramic mill.