Protein domains are distinct functional and structural units within a protein. Each domain can fold independently and often has a specific role, such as binding to other molecules or catalyzing reactions. These domains are typically composed of a sequence of amino acids that form a stable three-dimensional structure. Proteins can contain multiple domains, allowing them to perform various functions simultaneously. The study of protein domains is essential in fields like biochemistry and molecular biology, as understanding these units helps researchers decipher how proteins work and interact within living organisms.