  This article concerns the correspondence between thermodynamics and the
morphology of simple fluids in terms of clusters. Definitions of clusters
providing a geometric interpretation of the liquid-gas phase transition are
reviewed with an eye to establishing their physical relevance. The author
emphasizes their main features and basic hypotheses, and shows how these
definitions lead to a recent approach based on self-bound clusters. Although
theoretical, this tutorial review is also addressed to readers interested in
experimental aspects of clustering in simple fluids.
