Top Soil is the upper, outermost layer of soil, usually the top 2 inches to 8 inches. It has the highest concentration of organic matter and microorganisms and is used on top of fill dirt (for example, sod, planting).
Fill Dirt is earthy material that is used to fill in a depression or Sink hole in the ground or create mounds or otherwise artificially change the grade or elevation of real property. Fill dirt is usually subsoil (soil from beneath the topsoil) and underlying soil parent material, which has little soil organic matter or biological activity. Fill dirt is taken from a location where soil is being removed as a part of leveling an area for construction; it may also contain sand, rocks, and stones, as well as earth.