In soil science the various distinct strata within the soil are referred to as horizons. Some strata are very common, and are particularly useful in determining soil classification. It is important to keep in mind that a soil classification does not refer to 'sandy' or 'loamy', but refers to a layered sample involving multiple horizons, which may extend from a surface layer of compost all the way down to the bedrock, passing through any number of horizons along the way.
The B horizon is generally the third soil horizon down from the surface. Most of us would recognize it by the term subsoil. It lies beneath the O horizon and the A horizon, which are comprised of organic matter in various states of decomposition. In contrast, the B horizon is generally mineral in makeup. It is still fine-grained, usually being comprised of clay, sand, or sometimes volcanic ash. At the depth where significant amounts of parent material are present (i.e. rocks), it will be classified as the C horizon.
While the B horizon is primarily derived from the local parent material, it is common to find materials, such as iron, aluminium oxides, gypsum, and silicate clay that have leeched into the soil with the absorption of rainwater. This illuvium often includes organic matter from the upper horizons, so in some cases the B horizon can be relatively nutrient rich. Because this layer often traps leeched materials, it is sometimes called the illuviated horizon or the zone of accumulation.
While many plants will send their roots down into the B horizon, there is comparatively little life to be found here. Some burrowing animals and a smattering of bacteria may be found, but we are now below the biomantle. Together the O, A, and B horizons make up the solem.