A chinois is a conical sieve with an extremely fine mesh. It is used to strain custard, purées, soups, and sauces, producing a very smooth texture. It can also be used to dust pastry with a fine layer of icing sugar.
A related but far less expensive utensil, is the China cap, a reference to the conical straw hats common in China. It is a conical strainer of perforated metal with much larger holes than a chinois. It is used to remove seeds and other coarse matter from liquids and soft foods, but does not produce a very smooth texture.