Poaching is "to cook an item by submerging it in a liquid that is just barley simmering." Poaching is not a rolling boil. Poaching, compared to boiling, is a much gentler technique.
Poaching generally calls for food to be fully submerged in a liquid that is kept at a constant and moderate temperature, between 160° and 180°F. Keeping the temperature constant can take a little practice. The surface of the liquid should just shimmer with the possibility of a bubble.
The liquid is generally well flavored - stock, broth, court bouillon infused with herbs, spices or anything the imagination can conceive.
Usually the most delicate of foods, like eggs, fish, fruit, and some organ meats are poached. The food must be completely submerged in the water.
185 degrees to 200 degrees F.
Simmering is usually reserved for tougher cuts or items that need more time to cook. The temperature of the liquid is usually between 185° and 205°F. A simmer is sometimes called a "gentle boil." Small bubbles periodically rise to the surface - the gentler and slower the bubbles, the lower the temperature.
NOTE: You can simmer with a lid, but remember the temperature inside the pot will rise and the simmer can very easily turn into a boil.
The simmered item renders a broth that is served as the sauce with your dish.
212 degrees F.
Boiling food is the process of cooking it in a boiling liquid, usually water. Boiling water has a temperature of 212°, and no matter how long it boils or how hard it boils, it never becomes hotter; for at that point it is transformed by the heat into steam, and in time boils away.
This temperature varies with the atmospheric pressure, which in turn varies with both altitude and weather.