Galaxy clusters are the largest structures in the Universe. They are very interesting since they can be considered as galaxy laboratories or cosmological probes. In this talk, I will provide an overview of galaxy clusters from both points of view. I will first introduce the role that galaxy clusters play within the large scale structure and show recent results on cluster detection in a variety of surveys. Then, I will present on-going work on forecasting cosmological constraints from cluster counting, in particular, using mock catalogues that mimic next-generation surveys. Finally, I will summarize recent results on evolutionary process of brightest cluster galaxies (BCGs) within the last 7 Gyrs.