We commonly refer to both colanders and sieves as “strainers,” although technically we use a colander to drain (discarding liquids like pasta water) and a sieve to strain (saving liquids like broth for stock). A colander has a wide bowl (often with two handles) and feet or a base that let it stand on its own in a sink while you pour a pot of pasta or boiled vegetables into it. Made from plastic, stainless steel, aluminum, or enamel-coated porcelain, colanders usually have a lot of small holes regularly spaced all over, although some are made of mesh. Better models have plenty of holes close to the bottom of the bowl to prevent liquid from pooling up.
Sieves, on the other hand, are made of wire mesh and are designed with one long handle. The better ones have a hook or a loop that rests on the rim of a pot or bowl, making it more convenient to collect strained liquid. The bowl of a sieve can be rounded or cone shaped.
Sieves are usually referred to as coarse- or fine-meshed. You’ll find yourself using a coarse-mesh sieve for most everyday tasks, from straining small amounts of stock to sifting flour or other dry ingredients. When it comes to making exquisitely clear consommé or a very refined sauce, you will want a fine-mesh sieve—one that eliminates all lumps or any hint of graininess. A fine-mesh sieve can also be used for making smooth purées from cooked vegetables or fruits.
A classic conical shaped French sieve, called a chinois (pronounced sheen-WAH), has a double layer of fine mesh (which can withstand the pressure of a wooden spoon or pestle forcing solids through it) and is generally considered the Rolls-Royce of sieves.