Link to a picture of a soil horizon; better than the one in Ricklefs:
Characteristics of Major Soil Horizons (taken from Ricklefs)
O: Primarily dead organic litter. Most soil organisms inhabit this layer.
A: Layer rich in humus, consisting of partly decomposed organic material mixed with mineral soil. (A for …?)
E: Region of leaching minerals from the soil. Because minerals are dissolved by water in this layer, plant roots often concentrated here. Eluviation refers to the downward movement of dissolved or suspended material within the soil by leaching.
B: Region of little organic material, whose chemical composition resembles that of the underlying rock. Clay minerals and oxides of aluminum and iron leached out of the overlying E horizon are sometimes deposited (brought) here.
C: Primarily weakly altered material, similar to the parent material. Calcium and magnesium carbonates accumulate in this layer, especially in dry regions, sometimes forming hard, impenetrable layers or "pans."
R: Unaltered parent material, such as rock.
Every soil type will have these layers (with the exception of E, which does not always exist). Additionally, there are four major soil orders, representing various ages and stages of soil development with differing proportions of each soil type.
Entisol: Is rec"ent"ly developed soil. Weathering hasn't really started, so there are only small layers of O, A, and C, with a dominant bottom layer of R.
Inceptisol: Marks the "inception" of soil formation. Layer B appears, layer C grows larger, and layer R becomes smaller.
Alfisol: Is h"alf"way weathered. Layers B and C continue to grow, as layer R continues to shrink.
Ultisol: Is the "ult"imate result of weathering. In this final stage, layer R has disappeared completely, layer E has appeared, and layers B and C have continued to grow.