Protein
Much like carbohydrates and fats, protein is a macronutrient that can provide the body with energy (4 calories per gram). Unlike carbs and fat, protein is also used by the body as the building block for all tissue. Anytime your body has to make new tissues or repair damaged ones, protein is used as the raw material.
Protein's Role
Protein is the building blocks of all the tissue in the body. Hair, nails and muscle are all made from various proteins. Whenever you exercise or do any form of intense physical activity, you create tiny tears in your muscle tissue that when repaired, make you stronger, faster and bigger. Protein is used to repair muscle. Think of protein as the material that is used to fix body tissue while fats and carbs provide the energy or man power to do it.
Amino Acids
Protein is made up of amino acids that are linked together. There are two main types of amino acids, essential ones and non-essential ones. Essential amino acids cannot be produced by the body and must be brought in through the diet. Non-essential amino acids can be made by the body and it is therefore not required in the diet.
Complete vs. Incomplete Proteins
There are two main types of proteins, complete and incomplete. Complete proteins are those that contain all of the essential amino acids while incomplete proteins lack one or more eggsof the essential amino acids. This is important because for the body to build or repair tissue, it needs all of the essential amino acids. Does this mean you have to always eat complete sources of protein, not necessarily.
When a protein is digested, it is split up into the individual amino acids that it's made out of. The body then synthesizes new proteins out of the amino acids to suit its needs. If you have a balanced diet that includes a wide range of foods, your body will build complete proteins from incomplete sources. For example if one food contains amino acid X but lacks amino acid Y, eating it with a food that contains Y but lacks X will allow your body to take X from food # 1 and Y from food #2.
Sources of Protein
Complete proteins are found in animal products. They include dairy (milk, cheese, yogurt) meat, eggs and fish. Soy is the only vegetable protein to be complete. Protein supplements are also complete sources of protein and can increase your calorie intake if you're trying to gain weight.
How much protein do I need to eat each day?
Protein's main function in the body is to build and repair tissue. The amount of muscle tissue that needs to be repaired is not dependent on how much protein you eat, but rather the amount of damage you cause during your workouts. Generally, the more you workout (to a point), the more muscle you build. Eating more protein does not necessarily lead to more muscle, it can however lead to excessive body fat.
Source: http://straighthealth.com/pages/qna/what-is-protein.html