Primary Succesion=This is the development of a community where there was nothing before.  In this picture of a Maine landscape, the rock is growing lichens and algae, which will make small cracks in the rock.  The small cracks will collect some organic material, and then make larger cracks.  These larger cracks will collect much more material and establish a base for grasses and shrubs to grow.

Secondary Succession=This is development where a community was formerly present.  The above picture shows lake succession, a variation of secondary succession.  In this example, the floating vegetation creates a medium for shrubs, and sometimes even trees to grow on.  The acid they produce keep things from decomposing within the bog.