Proteins are made up of amino acids which are strung together in complex formations. Because of their complex structure the body takes longer to break them down. Hence they are digested more slowly and are longer-lasting source of energy as compared to carbohydrates.
Human contains 20 amino acids---some of these are made in the body but some are not made. The amino acids that are not synthesized in the body are called essential amino acids. These amino acids have to be provided from the food you eat. If your diet lacks good quality protein your body may develop deficiency of these amino acids. Milk, eggs, poultry, meat and fish are good source of proteins.
Proteins are building block in the body and are needed for growth, repair and maintenance of all most all cells of the body like the muscle, connective tissues, and skin. If you eat fewer calories than you need then your body will use protein for energy and not for growth. If you eat more protein than you need then the body stores the excess proteins as fat in the body.
Adults doing average activity need about 60 grams of protein per day or about 0.8 grams per kilogram of weight. Your 10 to 15% of total calories should come from proteins. Adults involved in competitive sports or body building, need more proteins. Similarly children need more as they are actively growing.