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 chemical reactions. These domains are typically composed of a specific sequence of amino acids that determine their shape and function. Proteins can contain multiple domains, allowing them to perform various tasks within a cell. For example, a protein might have one domain for binding to DNA and another for interacting with other proteins. Understanding protein domains is crucial in fields like biochemistry and molecular biology, as they help explain how proteins work and interact in biological systems.