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 C horizon is essentially the transition between the B horizon, which is generally comprised of sand and clay, and the R horizon, which is more usually known as bedrock. It is characterized as a mix of granulated soil and parent material -- generally fragments of country rock. It shows little or no evidence of active weathering, and while roots may intrude down to this layer, there is little activity here. In some cases karst weathering may take place, in which acidic water slowly melts away carbonate rocks such as limestone over the period of thousands of years, resulting in irregular lumps and pits sometimes called ghost rock formations. The C horizon may also be composed of heavily fractured bedrock, saprolite, partially cemented sediment.
The exact definition of the C horizon changes from country to country, but as a general rule of thumb, if it does not clearly meet the conditions for B horizon or R horizon materials and it lies between those two horizons, you are probably safe in calling is the C horizon.