Mosses are non-vascular plants; they cannot transport fluids through their bodies. Instead, they must rely on surrounding moisture to do this job for them. They lay the foundations for other plant growth, and prevent erosion.
Mosses, or byrophytes, are categorized into three classes: peat mosses (Sphagnopsida), granite mosses (Andreaopsida), and true mosses (Bryopsida or Musci). Mosses used to be classified as three classes of a single phylum, Bryophyta. Modern texts, however, now assign each class to its own phylum: mosses (Bryophyta), liverworts (Hepatophyta), and hornworts (Anthoceraphyta). This reflects the current taxonomic wisdom that the liverworts and hornworts are more primitive and only distantly related to mosses and other plants.