Explore the fundamentals of galaxy clusters in this comprehensive lecture by Stefano Borgani. Delve into the importance of these massive cosmic structures, their formation through vertical collapse, and the concept of yellow clustering. Gain insights into the historical context of cluster studies and the current state of research in this field. Examine key examples and engage in discussions about cluster characterization. Learn about the application of the Jeans Equation in understanding galaxy cluster dynamics. This lecture is part of a broader program on cosmology, covering topics such as dark matter, dark energy, and large-scale structure of the universe, organized by the International Centre for Theoretical Sciences.