This book chapter provides a comprehensive exploration of Organic Light Emitting Diodes, tracing their journey from fundamental materials to a myriad of applications that continue to shape the future of technology and design. It offers a valuable resource for researchers, engineers, and enthusiasts seeking to understand the intricate interplay between materials, physics, and practical implementations in the OLED ecosystem. Organic Light Emitting Diodes (OLEDs) have emerged as a transformative technology with a profound impact on various industries, ranging from consumer electronics to lighting and healthcare. This book chapter tells about the fascinating world of OLEDs, tracing their evolution from the early development of organic materials to their diverse and innovative applications. The chapter begins by providing an overview of the materials that form the foundation of OLED technology, elucidating the critical roles of organic semiconductors, emissive layers, and charge transport materials. Moving beyond the materials, the chapter delves into the core principles of OLED operation, elucidating the physics governing electron-hole recombination and light emission. It discusses the various OLED device architectures, including passive matrix and active-matrix configurations, shedding light on their advantages and limitations. A significant portion of the chapter is dedicated to OLED applications, demonstrating the versatility of this technology. It showcases OLED's success in display technologies, such as smartphones, televisions, and wearable devices, where their vibrant colors and flexibility have redefined visual experiences. Furthermore, it explores emerging applications in lighting, where OLEDs offer energy-efficient, aesthetically pleasing and tunable illumination.