High Entropy Alloys (HEAs) have recently attracted the researchers' attention as a new track in materials design. Research on HEAs has stimulated new ideas and inspired the exploration of the enormous composition space offered by multi-principal element alloys. HEAs can be simply described as alloying systems that are composed of multi-elements in equimolar or near equimolar ratio. Therefore, HEAs have unique properties which significantly differ from the traditional alloy systems. The current review article is concerned with the definition of HEAs, their development history, and their four core effects which make them different from the traditional alloys. Fabrication routes of HEAs with special emphasis on processing via casting technique are surveyed. The innovative microstructure of HEAs that lead to extraordinary mechanical properties is also discussed. Finally, the promising industrial applications and the future insights of these multi principal alloys are covered.