Soil is naturally removed by the action of water or wind: such 'background' (or 'geological') soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil. Even before this, natural processes moved loose rock ('regolith') around on the Earth's surface (just as has happened on the planet Mars).
Overall, background erosion removes soil at roughly the same rate as soil is formed. But 'accelerated' soil erosion — loss of soil at a much faster rate than it is formed — is a far more recent occurrence.
Background rates of soil erosion aren't a problem. But accelerated erosion is. Unsustainably high rates of soil loss are always a result of mankind's unwise actions, such as overgrazing or unsuitable cultivation practices. These leave the land unprotected and vulnerable. Then, during times of erosive rainfall or windstorms, soil from the fertile upper layer ('horizon') may be detached, transported, and — after possibly travelling a long distance — deposited elsewhere.
Because soil is formed slowly, it is essentially a finite resource. So soil which is removed by erosion from the upper horizons ("the cream of the soil") is, for all practical purposes, lost forever. But accelerated erosion does not only have impacts which are on-site (at the place where the soil is detached). It can also cause environmental problems off-site (wherever the eroded soil ends up).
And in recent decades, the use of powerful agricultural implements has, in some parts of the world, led to damaging amounts of soil moving downslope merely under the action of gravity. This is so-called tillage erosion.
Unsustainable soil loss by water, wind, or tillage is one of the most widespread of today's environmental problems, potentially affecting both agricultural areas and the natural environment. But the severity of the global erosion problem is only now becoming widely appreciated.
Here at The Soil Erosion Site, our focus is on soil erosion as a problem, i.e. on accelerated erosion. So on the remainder of the website, we mostly don't bother with the word 'accelerated'.
Dr David Favis-Mortlock, April 2017