Abstract: How to impartially quantify the scientific impact has always been a significant issue and attracted much attentions. To solve this problem, the H-index and its dozens of derivative indicators have been proposed. In this paper, we review H-index and many of its representative variants. We apply the Citation Distribution Curve of scholars or organizations as a unified perspective to discuss the essences of these representative indices and compare their advantages and disadvantages. Finally, we give an outlook for the future research directions.