Having revolutionized the smartphone and flat panel displays industry, Organic Light Emitting Diodes (OLEDs) have already become a multibillion dollar industry with more and more famous companies investing in this technology for their applications. Research in this area is growing at a fast rate resulting in different areas such as chemistry, physics and engineering to work together in order to improve the devices' efficiencies, power saving and overall simplicity. In this review, the basic mechanism inside an OLED is analysed and the basic characterization (electrical and optical) overviewed.