As endpoints of cosmic structure formation that emerge in the era of dark energy domination, the population of massive halos that house clusters of galaxies offers insights into the nature of cosmology and gravity. Astronomical survey teams are well are their way to mapping the cosmography associated with clusters across nearly the entire visible universe, with sample sizes soon approaching one million objects. The composition of dark matter and baryons in multiple phases within massive halos is being scrutinized across the electromagnetic spectrum and with increasingly sophisticated numerical simulations. In this talk, I outline the phenomenological framework of cluster cosmology, emphasizing multi-wavelength population statistics and support from astrophysics-based simulations. New data emerging from ongoing optical and X-ray surveys are presented, and challenges discussed, particularly the central issue of total cluster mass calibration.