Ecologically and structurally, mosses are closer to lichens than they are to other members of the plant kingdom. This affinity can be attributed, in part, to their structural similarities: both lack a vascular system for transporting fluids and therefore depend upon external moisture to transport nutrients to all parts of the plant.Mosses are the second line of attack in Nature's war against rocks. After lichens have created a foothold in rocks the mosses move in, ultimately becoming a layer of topsoil for higher plants to take root.
As with lichens, mosses are often overlooked by scientist and laypersons alike. Many field guides and even college texts limit themselves to the higher plants, leaving the moss-seeker helpless in discovering more about these lush green carpet-plants.
The moss Phylum (Bryophyta)is divided into three classes: True Mosses (Musci), Liverworts (Hepaticae), and Hornworts (Anthocerotae)
True Mosses (Class: Musci)
All plants reproduce through alternating generations. Nowhere is this more apparent than in the mosses. The "first" generation, the gametophyte, forms the green leafy structure we ordinarily think of as moss. It produces a sperm and an egg (the gametes) which unite, when conditions are right, to grow into the sporophyte or spore-bearing structure. The sporophyte ususually looks like a capsule on the end of a stalk (above left) and grows parasitically on the gametophyte. As the sporophyte dries out, it release sporres which will grow into gametophytes if they germinate.
In other plants the gametophyte phase is either inconspicuous, ephemeral or entirely hidden from sight; the second generation may produce seeds rather than spores.
Mosses do not bear true flowers but the gametophyte may sometimes produce a flower-like structure with male and/or female sex parts as in the Juniper Haircap moss (Polytrichum juniperinum) pictured above right and below.
True mosses are the most common, diverse and advanced brypophytes. 
Order: Polytrichales; Family: Polytrichaceae
Liverworts (Class: Hepaticae)
The liverworts can be leafy like true mosses or papery and flat like a foliose lichen.
The Marchantia polymorpha pictured here is of the latter type and is distinctive in having cup-like holders for disk-shaped gemmae. The gemmae allow the gametophyte liverwort to reproduce asexually.
Liverworts produce sporophyte generations too but the sporophytes are usually short-lived and difficult to find in the field.
Order Marchantiales, Family Marchantiaceae
Hornworts (Class: Anthocerotae)
Hornworts are very similar to liverworts but differ in the shape of the sporophyte generation. Instead of generating spores in capsules on top of a stalk, the hornwort generates spores inside a horn-like stalk. When the spores mature the stalk splits, releasing the spores.
Under the microscope, hornwort cells look quite distinct as well: they have a single, large chloroplast in each cell. Other plants typically have many small chloroplasts per cell. This structure imparts a particular quality of color and translucency the body (thallus) of the plant.