Protein is the body's dietary tool box. It extracts proteins, breaks them down into usable forms, and applies them where needed. The dietary process provides the body with a range of amino acids for building new cells and related genetic functions. It's also an energy source, and often used as an efficient way of providing body mass in commercial diets.
At the molecular level, proteins are sources of other nutrients like nitrogen, used in all areas of the body as part of the basic CHN (carbon, hydrogen nitrogen) structure of every type of body cell.
Proteins occur in two basic forms, animal protein and vegetable protein. These are quite different forms of proteins. The general composition of the proteins means that one unit of animal protein contains 4 times the amount of protein than vegetable. However, vegetable protein also contains a wide range of very useful materials, including a lot of nitrogen, so both types of protein are valuable. Humans are omnivores, able to source protein from a variety of sources, and the medically preferred dietary balance contains a lot of both types.
The value of dietary proteins is the basis of protein diets. Excessive protein intake puts strain on the liver and kidneys, and excess protein isn't retained by the body. The different types of protein also have varying levels of availability when ingested, so dietary considerations aren't quite as simple as simply eating the protein and getting the full value of the amount eaten.
Note: Natural sources are far better overall as dietary adjustments. Real food is the best source of protein. Body mass related protein diets are highly debated among nutritionists. The typical protein supplement quality varies considerably. Many contain large quantities of sugars and carbohydrates, which are ephemeral dietary supplies, adding weight but then losing it unless the dosage of supplements is maintained. To avoid getting ripped off, check protein supplements for sugar content. One of the best possible protein diets is to remove fats from meat.