What is speciation?

Speciation is the EVOLUTION of a NEW SPECIES from an EXHISTING SPECIES. 

If two populations are seperated so the flow of alleles stops (e.g geographic isolation- a physical barrier that prevents the 2 from breeding, aka mountains or oceans) and enviromental differences causes the two populations to evolve seperately by selection, then the two populations will evolve and eventually will not be able to breed together anymore.