So, you've decided you want to get ripped. Huge. Swole. Muscly. So you go out and buy a huge vat of protein powder, drink one twice a day, and you're set, right? Not even close.

Protein is essential for building muscle, but it does so much more than that. Without proteins, your body would fall right apart. And if you think that you're building muscle by ingesting more protein, chances are all you'll gain is some extra fat.
What is Protein?
Food with protein
The word "protein" is simply a collective term for a gathering of many individual protein molecules. The plural "proteins" is a more precise way to refer to them, because there are many, and we need almost all of them.
Take a length of flat, skinny ribbon about six feet long, and crumple it up in your hand - that's a huge-scale representation of a protein molecule. They are large. They are complex. And their job is to rebuild the cells (and by extension, tissues) that are constantly breaking down in your body.

Your muscles need protein, but so do your organs and your blood. Without proteins, we wouldn't have antibodies, so we'd be at the mercy of every bacteria, fungus and virus around.
Amino Acids
Amino acid
Proteins are made up of individual units called amino acids. There are a bunch of different amino acids, but 22 in particular are vital to the human body.
Amino acids are divided into three groups:
Essential amino acids are the ones we need to get from food everyday. There's no need to worry about the individual aminos themselves, because as long as you eat things that come from animals, you get them all - animal proteins like meat, poultry and fish are called "complete proteins", which means they contain all the essential amino acids.
Plant proteins (except soy) are incomplete, meaning they contain some aminos but not others. This varies from plant to plant, so vegetarians must eat a wide variety of plants to ensure sufficient protein intake.
Non-essential amino acids are deemed "non-essential" because our body makes them itself from the raw materials we give it when we eat the essential aminos. The non-essential amino acids are just as important to our health, we just don't have to worry about eating them every day because our bodies take care of that for us.
Conditional amino acids are a smaller group, and they only become important under times of stress. Mental stress sometimes counts, but sickness or injury is when your body really needs them - remember, protein's job is to repair tissue, so it's very important when recovering from an injury. Antibodies also need proteins to help ward off infection.
What's the Best Protein to Eat?
Protein diet
Many people think in terms of amino acids when thinking about protein foods, and this is sort of irrelevant. As long as you're eating enough overall protein, your body is getting what it needs. When you digest protein foods, your body breaks them down into amino acids. When there is a need for a particular protein somewhere in the body, your body gathers the aminos necessary to build that protein and assembles it on the spot.

A better way to think about it is bioavailability, or how much protein your body is actually able to absorb from a particular protein food. Soy works best for vegetarians. Eggs, poultry and fish too, are good sources of protein. But don't go crazy - there's a theory that the body can only absorb about 30 grams of protein per meal anyway. Don't waste your money on a protein shake that promises 250 grams of protein per serving, because you'll just be creating expensive urine.
Muscle Building
Man drinking protein shake
So why won't a protein shake give you muscle if their job is to repair muscle? Because "repair" doesn't mean "spontaneously create". First you have to injure the muscle tissue you wish to grow (just a little). That's why people lift weights - when you lift something heavy, it causes a bit of damage to your muscles. Protein rushes in and repairs the damage, and adds a little extra so you're better able to handle the load next time - that's how muscle growth happens. Slowly, bit by bit, and only under repetitive stress.

The reason why the protein shakes might make you fat is a simple question of calories. If you consume more calories than you expend, your body stores them as fat. Now, your body needs a few extra calories to provide the energy to build the additional muscle tissue, but 250 extra calories per day is really all. If you have any extra fat on you, you don't even need that. And you only need those calories if you're working out consistently.
So in the end, yes, proteins are extremely important. But a protein shake is not the magic answer to your skinny arms and sunken chest. Get your protein from real food, and count on a daily heavy lifting routine to see the changes you're looking for.