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What's the main difference between primary ecological succession and secondary?

Primary Succession
Gradual growth of organisms in an area that was previously bare, such as rock. (For example lichens, mosses, and ferns will first appear on bare rock).
Secondary Succesion
It is the series of community changes which take place on a previously colonized, but disturbed or damaged habitat, (examples include areas which have been cleared of existing vegetaion such as a tree-felling in a woodland).
Compare && Contrast
Primary succession occurs following an opening of a pristine habitat, for example, a lava flow, an area left from retreated glacier, or abandoned strip mine.
In contrast, secondary succession is a response to a disturbance, for example, forest fire, tsunami, flood, or an abandoned field.