How do erosion and deposition work together to form sand dunes?

The wind moves sand particles through the process of eolic erosion, from one place to another. When particles meet an obstacle, they deposit. The sand covers the whole obstacle. A new dune is formed. The dune has a soft slope directed to the side from where the wind erosion it, and a sharp slope from the opposite side, where the wind loses strength and particles fall depositing.  

Explanation:        

Winds can transport many different-sized particles of material from the ground through the process of erosion. These particles might travel many kilometers until they deposit somewhere. Regions with no vegetation have too much dust and grain-sized particles that move and crush with each other, increasing erosion. This process is known as eolic erosion. Dunes are winds deposits, formations that are typical of deserts. Dunes are orientated according to the direction and speed of winds, to the presence of loose material, and vegetation. Strong winds transport sand until it collides with an object or obstacle where it is deposited, originating a new dune.

Dunes are very dynamic formations that are constantly being remodeled by the direction and strength of the winds. Dunes always have a soft slope and a sharp one. The soft slope is directed to the area or region where the wind is coming from and is the product of the wind erosion on the same dune. The opposite sharp slope is protected from the wind and is the deposition place, where the wind losses strength and the carrying material falls, depositing in this side of the dune.

This is how erosion and deposition work together to form a dune.