Protein is the building block of our whole body - Essential M muscles, tissues, hair, nails, etc - all made up of protein. The body needs to break protein down to amino acids to build muscles. If you do not have enough protein in your body you can NOT build muscle mass. You want to make sure that your body is burning fat, not muscle and you can ensure this by ingesting .9 grams of protein per pound of body weight. If you want to build muscle, you will need to ingest 1 to 1.5 grams of protein.
Most protein foods of animal origin such as meat, poultry, fish, eggs, cheese, and milk, contain all of the essential amino acids and are called complete proteins. Proteins found in vegetables are usually lacking one or more of the essential amino acids, so they are called incomplete proteins. However, you can combine 2 incomplete vegetable proteins to make a very high quality complete protein. These are the foods are where most of your protein grams should come from first.