The best way to describe co-evolution is with specific examples like the orchid/moth example from the wiki page.
If a species of moths were the primary pollinators for the orchid, and a different selective pressure caused orchids with deeper flowers to be more fit and likely to survive, the moth would then have a selective pressure to have a longer tongue to be able to better get the nectar for food and reproduce.
The pressure on the orchid caused an evolutionary change that also forced a change on the moth. (2 steps) They co-evolved (orchids and moths together), but all of the orchids and all of the moths evolved in the same way so there is still just 1 orchid species and 1 moth species, but they are phenotypically different from their ancestral forms.
Speciation would be a single species eventually becoming two separate species because of isolation or a selective pressure on certain individuals of the population. (1 step) An example would be if medium sized individuals were less fit than small and large individuals (speciation through divergent evolution). At the end you have 2 new species from the original population.
