<p>Conformational entropy, one of the central concepts of polymer physics, is the key to revealing physical characteristics of polymers. Despite an increased repertoire of conformational-entropy effects in the structural formation, transition, and properties of polymer systems, the physical origin of conformational entropy remains less understood compared to interaction energy and other types of entropy. This review seeks to provide a conceptual framework unveiling several principles and rules of conformational entropy in governing the structures and properties of polymers, from the perspective of fundamental physics and statistical mechanics. First, we focus on the fundamentals of entropy in thermodynamics, leading to the theoretical basis for the elucidation of conformational entropy. Second, we delineate the physical nature of statistics and dissipation of conformational entropy and its essential dependence on the environmental heat bath. Next, we explore the principles of conformational entropy in driving the ordering transitions of various systems of polymers and their nanocomposites, elucidating the emergent and collective behaviors as well as the interplay between energetic interactions and entropy. Moreover, we demonstrate how the concept of conformational entropy is generalized to the biological systems and other soft matters. Finally, we discuss future directions to signify this framework originated from polymers.</p>