What is the difference between Topsoil and Fill Dirt?

Topsoil is a sandier, fertile, usually dark brown or black, soil that contains nutrients and organic matter suitable for growing plants.  Fill dirt refers to any dirt that does not contain organic matter but instead contains higher amounts of clay.  Some fill dirt may be suitable for compactible fill.  Fill dirt is usually a more economical option and better option to fill in low areas for structured fill.  Topsoil is best used for areas that will be planted with grass, trees, or shrubs.