There are nine "essential" amino acids—they are called "essential" because our bodies can't produce them, so it's essential that we include them in our daily diet.
When we eat protein, it is broken down in our gastrointestinal tracts into individual amino acids and then put back together again as new proteins. This complex biological process is called protein biosynthesis.
Plant proteins rich in essential amino acids include combinations of whole grains, soybeans, lentils, chickpeas, black beans, pinto beans, almonds, walnuts, and sunflower seeds. Of these, only soybeans are considered a complete protein. Other plant proteins are considered "incomplete" because they contain only some essential amino acids.
Animal proteins rich in essential amino acids include beef, pork, turkey, chicken, eggs, milk, yogurt, cheese, and seafood. Animal proteins are considered "complete proteins" because they contain all of the essential amino acids.