What is the difference between dunes and loess?

Sand dunes are made from mainly one mineral - sand or quartz grains and they form and move to develop a typical cross bedding structure.

Loess however is wind blown silt (a mixture of sand and clay) with a fine grain size typically in the 20-50 micrometer range. The deposits are typically non stratified and cover a wide area. They also quickly become loosely cemented after deposition and are not reworked as are sand dunes.

The source material for loess deposits is the rock flour left by retreating Ice sheets and the deposits therfore form in temperate regions while Sand Dunes are an indication of arid desert (tropical) conditions.