With subsidence the supporting soil moves away from the structure taking away its support and allowing the structure to move. Settlement is where the structure moves the soil due to application of load.
Settlement normally occurs early in the life of a property whereas subsidence can occur at any time. In a large part of the South East of England we have large areas of Clay Subsoil and movement of this is the main cause of subsidence problems. Subsidence is normally covered by insurance whereas settlement is not.