Once you start looking at mosses closely, they sort into two broad camps, and learning the difference is one of the most useful things a beginner can do. It explains why some mosses make tight domes and others spread in flat carpets, and it quietly decides which moss is right for a lawn, a wall, a terrarium or a bonsai.
The two forms
The two camps are the acrocarpous mosses and the pleurocarpous mosses. Acrocarps grow upright in tufts and cushions, with stems that mostly stand on end and grow from the tip. Pleurocarps creep and trail, with much-branched stems that spread sideways into mats and wefts. The cushiony bun moss on top of a wall is a classic acrocarp; the flat, feathery plait moss trailing over a log is a classic pleurocarp.
How to tell them apart
The names come from where the spore capsules grow, and that is the surest test. In acrocarpous mosses the capsule forms at the very tip of the main stem, which also tends to stop that stem growing upward, so they stay compact and upright. In pleurocarpous mosses the capsules grow on short side branches along the stem, not at the tip, leaving the main stem free to keep creeping and branching. Even without capsules you can usually tell by habit: upright and tufted, or sprawling and feathery. A hand lens makes it obvious, as covered in how to identify moss.
Acrocarps in practice
Because they grow as discrete upright cushions, acrocarps hold a domed shape beautifully and are slow, tidy and sculptural. They are the stars of a terrarium, where their little hills give structure, and of Japanese moss gardens, where the rolling cushion texture is the whole point. The trade-off is that they spread slowly and knit into a continuous surface reluctantly, so they are less good where you want fast, seamless cover.
Pleurocarps in practice
Pleurocarps are the carpet-makers. Their creeping, branching habit lets them knit quickly into a continuous mat, which makes them the right choice for a moss lawn, a green wall or covering ground fast. They are generally more tolerant of being walked on and handled, and they fragment and regrow readily, which is why most slurry and propagation work uses them. The trade-off is that they look like a carpet rather than a collection of jewel-like cushions.
Why it matters for your project
Match the form to the job. For a lawn, a wall or any fast continuous cover, reach for pleurocarpous carpet mosses. For a terrarium, a moss garden or anywhere you want shape and texture, choose acrocarpous cushions, or mix the two, cushions for the hills and a pleurocarp to knit the floor between them. Knowing which is which turns "buy some moss" into choosing the right tool, and it is the most practical scrap of botany the whole subject offers. See the species page for common examples of each.