This chapter provides a comprehensive overview of the phylum Cnidaria, historically referred to as Coelenterata, with a focus on the biology, classification, and ecological significance of hard corals (Scleractinia). It begins by detailing the fundamental features of cnidarians, including their radial symmetry, biphasic life cycle, and specialized tissues, followed by a taxonomic breakdown into major classes and subclasses. Special emphasis is placed on Hexacorallia and its scleractinian representatives, which are the primary architects of coral reef ecosystems. Detailed anatomical and skeletal descriptions highlight the diversity in corallite morphology, colony growth forms, and reproductive strategies—ranging from asexual budding and fragmentation to complex sexual reproduction involving synchronous spawning. The chapter also explores coral symbioses, particularly with zooxanthellae, which underpin coral nutrition and reef productivity. Finally, it addresses the global distribution and structural variations of coral reefs, including fringing, barrier, and atoll types. Together, these insights underscore the intricate biology and ecological importance of hard corals as keystone species in marine ecosystems.