Image by NASA Globe program http://ltpwww.gsfc.nasa.gov/globe/pvg/texture1.htm

Aggregation
Particles of fine sand, silt and clay may join together to form aggregates. The soil property which describes the character and formation of these aggregates is called soil structure. The glue that joins the soil particles together includes organic matter, clays, iron oxides, aluminum oxides, and lime. Aggregate formation in clay-textured soil improves water infiltration into the soil and drainage because it increases the number of large pores (larger pipes). In sandy textured soil, aggregate formation reduces the excessively fast drainage of water by increasing the number of small pores (narrow pipes).

Aggregates can be described by their grade. The grade is a measure of how well the aggregate is cemented together or, conversely, how easily it is broken down by the impact of water, wind, or human activity. A soil with good structure has many aggregates present which are stable, meaning well cemented together. Such aggregates resist the forces of water, wind, and human activity. Consequently, they can maintain their porosity and allow better water and air movement.

Soil Moisture
Moist soils are less prone to wind erosion due to the cohesive or binding effects of water on soil clay and organic matter. As wind dries the soil, the risk of soil loss erosion by wind increases, especially if aggregate grades are weak. The risk peaks at or below permanent wilting point.