To simmer and to poach both involve cooking foods submerged in water or in a seasoned or flavored liquid. The two methods are almost identical except for temperature. The two methods can be summarized as follows:
Simmering
Cooking liquid is bubbling gently at a temperature of about 185° to 200°F (85° to 93°C).
Used primarily to tenderize tougher meats and poultry by long, slow cooking. Also used to cook some shellfish items, such as lobster.
Poaching
Cooking liquid is not bubbling but is barely moving, at a temperature of about 160° to 185°F (71 to 85°C).
Used primarily for cooking seafood and some tender cuts of poultry and meat, to retain moisture and develop a light, subtle flavor.