Boiling is used to prepare everything from pasta to green vegetables and stewed meats. To boil means to cook in a liquid that is bubbling,
When ingredient are boiled, they are done so in water, sometimes containing salt and oil or butter for flavor and texture. The food is usually added to the liquid once it reaches a boil.

Water boils at 212'F (100°C) at sea level. No matter how high the burner is turned, the temperature of the liquid will go no higher!

What boiling does:
Pasta: boiling water k keeps pasta in motion, prevents sticking and cooks quickly so it doesn't get soggy. 

Green vegetables: tossed into boiling water to o cook as quickly as possible will retain their flavor and bright color in a process called blanching.
reducing sauces: speedy evaporation is a useful effect where the volume of the liquid decreases and flavors are concentrated.
Meats, fish, and eggs: the high temperature toughens the proteins


Poaching is the cooking method of gently simmering food in usually a small amount of liquid, hot but not actually boiling
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 meats in order to eliminate undesirable flavors and to firm the product before final cooking.

To poached food, we use a poaching liquid called court bouillon. A classic court bouillon consists of:

An acid - wine, lemon juice
Aromatic - bouquet garnish
Poaching liquid - water, stock, chopped onions, carrots and celery.
The correct poaching liquid to be used to the type of food.

Here are the best practice for poaching:
The poaching liquid temperature should be around 160-180 F (71-82°C). It is important to keep the poaching heat low and to keep the time to a bare minimum, which will preserve the shape and flavour of the food. To serve poached chicken safely it has to reach an internal temperature of 165'F (74°C)

Broiling and Pan Broiling

To broil means to cook foods with high heat directly from the above. Broiler is a cooking method used mainly for tender meats, poultry, fish and some vegetables, and it's a popular in low-fat diets.
A low-intensity broiler called a salamander is used in a professional kitchen. It is smaller than a standard broiler and mainly used to finish off dishes, such as caramelizing the sugar on Crême Brülée, browning or melting the top before service.
Salamander - a low intensity broiler

Pan-broiling is more similar to griddling, except it's done in a sauté pan or skillet griddle surface. For example, broiling a very thin steak (minute steaks) to the rare stage is difficult because the heat is not high enough to form a good brown crust without overcooking the inside.  Pan-broiling in a heavy iron skillet is an answer to this problem. When pan-broil, fat must be poured off as it accumulates, or the process becomes a pan frying. No liquid is added, and the pan is not covered, or else the item would steam.

Pan Broiling

Procedure for pan broiling meats Preheat an iron skillet over a high flame until it is very hot. Do not add fat. The meat should be well seasoned. Proceed the same as for grilling. Pour off any fat that accumulates during cooking, if necessary. Properly broiled meats have a well-browned, flavorful crust on the outside, and the inside is still juicy.

Pork and veal are usually eaten well done, and generally better griddled, sautéed or braised than broiled. For best results, only high-quality, tender cuts with a good fat content should be broiled. Meat broiled to the point of being well done is likely to be dry. The best, juiciest broiled meats are those cooked to the rare or medium-done stag. Because of the intense heat, it is difficult to broil meats to the well-done stage and still keep them juicy. A steak that takes 10 minutes to broil may be cut and trimmed in advance, but broiling should be started 10 minutes before it is to be served.
It takes practice and experience to cook foods of different thickness to the right degree of doneness inside with the desired amount of surface browning.

Here are some rules to note:

Turn heat on full: Cooking temperature is regulated by moving the rack nearer/farther the heat Use lower heat for larger, thicker items and for items to be cooked well done. cooked to the desired degree at the same time.

Use higher heat for thinner pieces and for items to be cooked rare, so the inside and outside are Preheat the broiler to sear the product quickly and to make the desired marks on the food.

 Foods may be dipped in oil to prevent sticking and to minimize drying. This may not be necessary if the food is high in fat. Care should be taken, as too much oil on a hot broiler grate may cause a fire. Turn foods over only once, to cook from both sides and to avoid unnecessary handling.