A moss is a type of
primitive plant in the division
Bryophyta which does not have a
vascular system and lacks true
roots.
Peat mosses (Class Sphagnopsida) are plants which form spongy, pale-green mats in still freshwater environments like pools, bogs, swamps, and the shores of lakes and ponds.
Members of Class Andreaeopsida (which includes the granite mosses) are blackish-green plants with rhizoids which grow on silica-containing rocks (such as granite) at high altitudes and on soils in cold regions.
True mosses (Class Bryopsida) are a large and diverse group of plants that tend to consist of tiny, branching filaments, rhizoids, and reproductive organs at the tips of their leaflike filaments which release spores. They mostly grow on tree branches, rocks, and bare soil, but may also grow in freshwater streams or in arid regions.
From the science dictionary at http://biotech.icmb.utexas.edu/