This colloquium presents a simple, unified picture for the formation and evolution of star clusters. These objects are important in the structure and evolution of galaxies, primarily as the sites of star formation and stellar feedback. The colloquium is organized around the mass function of star clusters (i.e., the spectrum of cluster masses) and how it evolves with time (age). Observations show some remarkable similarities in the mass functions of young star clusters in different galaxies, analogous to the similarities in stellar initial mass functions (IMFs). Explaining the near universality of the mass functions of star clusters is one of the goals and successes of the theory presented here. A byproduct of this theory is a unified concept of star clusters of all types: associations, open clusters, globular clusters, etc. This traditional nomenclature has little physical content and should be abandoned. In the new picture, clusters are characterized simply by their masses and ages. The colloquium is aimed at a broad audience of theorists and observers, students and professors.
