Moss appears where conditions favour it over whatever you would rather have: shade, damp, poor drainage and compaction. You can scrape it off, but unless you change those conditions it returns. Here is how to clear each surface and, more importantly, keep it gone.
Lawns
Rake or scarify out the dead and living moss, then treat the cause: relieve compaction by spiking or aerating, improve drainage, cut less hard, reduce shade where you can, and feed the grass so it competes. A lawn sand or iron-sulphate treatment blackens moss quickly but changes nothing underlying. The full picture is on the moss and lawns page.
Roofs and gutters
Brush moss off dry, working downwards so you do not lift tiles, and clear it out of gutters and valleys where it traps water. Do not pressure-wash: the force strips the protective surface off tiles and slates and shortens the roof's life far more than the moss would. Improving light and airflow, and fitting zinc or copper strips near the ridge, slows regrowth.
Patios, paths and decking
For paving and stone, a stiff brush and a bucket of hot water shift most of it; a patio cleaner or a weak solution does the rest. On decking, scrub along the grain and rinse. Keep these surfaces clear afterwards by improving drainage and cutting back overhanging growth so they dry out between downpours. On steps, treat moss as a slip hazard and stay on top of it.
Chemical mosskillers, and when to skip them
The common chemical treatments are based on ferrous sulphate (iron), the active part of most lawn sands and path mosskillers. They work fast, blackening moss within a day or two, and on a lawn they have the side benefit of greening the grass. The catch is that they kill the existing moss without touching the reasons it grew, so on a shaded, damp surface you are signing up to repeat the dose every season. They also stain paving and concrete with rust marks, so on a patio or path a brush and hot water are usually the better tool. Bleach and harsh cleaners shift moss too but harm surrounding plants and soil life, and are best avoided outdoors.
Prevention that actually holds
Clearing moss is the easy half; keeping it gone is the half that lasts. Everything that helps comes down to letting the surface dry and see daylight: cut back overhanging branches and shrubs, clear leaf litter and debris that hold damp, improve drainage so water does not sit, and on roofs fit zinc or copper strips whose run-off discourages regrowth. None of this is dramatic, but a surface that dries out between downpours grows far less moss than one that stays perpetually shaded and wet.
The honest caveat
If a surface is permanently shaded and damp, moss will keep coming whatever you do. At that point the real choice is between clearing it on a schedule and simply accepting a little green. Away from steps and other places where a slippery film is a hazard, living with it is far less work and arguably better looking than a constant chemical battle.