The stages of ecological succession are primary succession, when opportunistic species move into a mostly lifeless environment, and secondary succession, when new species displace the primary succession species once the latter has modified the environment. In some cases, secondary succession proceeds to a climax community.
A climax community is a final, relatively permanent result of secondary succession. The species in a climax community have complex interactions and can, barring a natural disaster, out-compete any other species that tries to establish itself. Not all environments achieve a climax community. Often, small disruptions to the environment keep the species there in flux, so that at any time, one species or another can become dominant.
Primary succession only occurs after a natural disaster has cleared away life and well-processed soil from an environment. New lava flows, new sand dunes or bare rocks left behind by glaciers are all examples of environments suitable for primary succession. As the species adept at primary succession move in, they start to alter the environment by doing things like breaking down rocks, changing mineral compositions in soil or creating shade.
Secondary succession is not a single change in the species that occur in an environment, but all the transitions in species dominance that occur after primary succession.