What is ' Stock ' in cooking . .,

Stock is a flavored water preparation.
It forms the basis of many dishes, particularly soups and sauces. One of the first lessons in cooking would be is learning how to make stocks. Stocks form the basis of most sauces and soups. If you want to improve your cooking at home, you will want to learn how to make stocks and stock reduction technique.
Stock is made by simmering various ingredients in water, leftover cooked meat, Veal, beef, and chicken bones are most commonly used. Mirepoix is a  combination of onions, carrots, celery, and sometimes other vegetables.
The herbs and spices used depend on availability and local traditions. In classical cuisine, the use of a bouquet garni (or bundle of herbs) consisting of parsley, bay leaves, a sprig of thyme, and possibly other herbs, is common. This is often placed in a sachet to make it easier to remove once the stock is cooked. Today, ready made stock and stock cubes  in a  dried form are easily available.
The difference between broth and stock.
The terms stock and broth are somewhat interchangeable, but stock usually refers to the homemade variety, whereas broth could be  a canned one.
Stock is the thin liquid produced by simmering raw ingredients and  solids are remove.
Broth differs as  in that it is a basic soup where the solid pieces of flavoring meat or fish, along with some vegetables, remain.
Vegetable stock is made only of vegetables. It is common today.
Vegetable Stock is one that has reduced to be added  to soups and sauces, rices and grain dishes.
A few basic steps in making a stock are the  ingredients are simmered starting with cold water.
Stocks are simmered gently, with bubbles just breaking the surface, and not boiled.
Salt is usually not added to a stock. 
Meat is added to a stock before vegetables.
If making a vegetable stock then try this tip ...
Roast the vegetables before adding to the water kept on heat.