A complete protein contains all nine essential amino acids that the body cannot synthesize, making it vital for various bodily functions, particularly tissue repair and muscle health. Sources include animal products like meat, fish, eggs, and dairy. In contrast, an incomplete protein lacks one or more essential amino acids and is typically found in plant-based foods, like beans, nuts, and grains. However, combining different plant sources can create a complete protein profile, enabling vegetarians and vegans to meet their amino acid needs.