Each technique is defined by an approximate range of temperatures, which can be identified by observing how the cooking liquid behaves. Each one—poaching, simmering, and boiling—has certain telltale characteristics:
Cooking in liquid with a temperature ranging from 140 F to 180 F is called poaching and is typically reserved for cooking very delicate items like eggs and fish.
Within this range of temperatures, the poaching liquid won't show any visible signs of bubbling at all, though small bubbles may form at the bottom of the pot. This means that the best way of checking that the temperature is correct is with an instant-read thermometer.
With simmering, the cooking liquid is a bit hotter than poaching—from 180 F to 205 F. Here we will see bubbles forming and gently rising to the surface of the water, but the water still isn't at a full rolling boil.
Because it surrounds the food in water that maintains a more or less constant temperature, simmering cooks food very evenly. It's an excellent choice for culinary preparations including stocks or soups, starchy items such as potatoes or pastas, and many others.
One of the downsides to cooking this way is that it can cause the food to lose vitamins and other nutrients by leaching into the cooking liquid.
The hottest of these three stages is boiling, where the water reaches its highest possible temperature of 212 F. It's actually the least likely of the three to be used for cooking. That's because the violent agitation caused by the rolling boil can be too rough on food and will often damage it.
Water at a full boil would be a bad choice for cooking an egg outside its shell, as we do when preparing poached eggs because the agitation would cause the egg to fall apart. The same holds true for delicate fish as well as some pastas.