Dietary protein is a term used to refer any food or supplement source of protein. Protein is important for muscle growth and exercise due to the mass of amino acid required to build muscle, and also due to the activity of specific amino acids like leucine which can act to promote muscle protein synthesis.
For Maximal improvements in muscle growth and exercise performance, consume a sufficient amount of protein each day from food. If your food intake does not cover your protein needs, then supplemental protein such as whey or casein protein can be used.
Generally Healthy adults is 0.8g of protein per kg of body mass.
For an active person the amount increases to roughly 1.4-2.0g of protein per kg of body mass.
This means a person weighing 150 pounds would need between 95g and 130g of protein per day.
Research has suggested that higher amounts of protein in the diet may be vital for immune function, metabolism, satiety, weight management and performance.

Referenced by
Precision Nutrition
Muscle gain & exercise performance