To poach, to simmer and to boil all mean to cook food in water or seasoned and flavored liquid. The temperature of the liquid determines the method.
Boil means: to cook in a liquid that is bubbling rapidly and is gretly agited. Water boiles at 212F ( 100c ). Boiling is generally reserved for certain vegetables. The high temperature would toughen the proteins of the meats, fish and eggs and the rapid bubbling breaks up delicated foods.
Simmer means: to cook in a liquid that is bubling very gently. Temperature is about 185 t0 205 F (85 t0 96C). Most foods cooked in a liquid are simmered. The higher temperature and intense agitation of boiling are detrimental to most foods. The word "boiled " is sometimes used as a menu term, as when simmered fresh beef is called " Boiled beef "
Poach means : to cook in a liquid, usually a small amount, that is hot but not actually bubbling. Temparture is about 160 to 180F ( 71 to 82C) . Poaching is used to cook delicate foods such as fish and eggs out of the shell. It is also used to partially cook foods such as Variety meant, in order to elimate undesirable flavors and to firm up the product before final cooking.
Blanch means: to cook an item partially and very briefly, usually in water, but sometimes by other method (as when french fries are blanched in deep fry ).There are two ways of blanching in water.