The last few years have witnessed a growing amount of empirical evidence pointing to the existence of multiple stellar populations in some Galactic globular clusters. It is also becoming more and more clear that clusters, hosting multiple stellar populations, do share some common properties, but also differ from each other in various aspects. In this talk, I will review the recent results concerning the presence of multiple stellar populations in stellar clusters, emphasizing the (different) properties of the subpopulations in the various clusters, and how they have been interpreted so far. I will discuss also the global characteristics of "peculiar" clusters - hosting multiple populations - from different points of view, and compare them with "normal" clusters, to try and shed some light on their nature and origin.