Erosion is the detachment of earth material from the surface. Once detached, agents like water or wind transport the material to a new location where it is deposited. The most ubiquitous form of erosion is that done by water.
Rain splash erosion is caused by the impact of water striking the surface. Rain splash erosion generally takes place in two steps. As precipitation is absorbed by the surface it fills the pore spaces, loosening soil particles and driving them apart. The impact of subsequent rain drops hitting the surface splash the particle away from the point of impact. The effect is to give the surface a dimpled-like appearance.
Surface runoff forms when the rainfall intensity of a storm exceeds the infiltration capacity of the soil. Sheet erosion is caused by the unconfined flow of water running across the surface. The effects of sheet erosion are often hard to distinguish because such thin layers of soil are being removed. It isn't until several years later that significant degradation is perceived.
Rill erosion is caused by water concentrating into innumerable, closely-spaced small channels. Left unchecked, rills can cut vertically and horizontally and when joined form gullies.
Gullies are steep-sided trenches formed by the coalescence of many rills. Once started they are difficult to stop.