Protein structure is categorized into four primary levels: primary, secondary, tertiary, and quaternary. The primary structure refers to the linear sequence of amino acids, while the secondary structure involves local folding patterns like alpha helices and beta sheets. Tertiary structure describes the overall three-dimensional shape of a single polypeptide chain, and quaternary structure refers to the assembly of multiple polypeptide chains into a functional protein complex. Additionally, motifs and domains are important concepts that describe specific structural features and functional regions within proteins. Understanding these levels is crucial for grasping how proteins function in biological systems.