What is the difference between allopatric and sympatric speciation?

Allopatric speciation occurs when two species are living in separate environments and therefore there is no gene flow between the populations. This will then cause the populations to differentiate (change) because they will become adapted to the different environments and niches they are living in. Sympatric speciation occurs when two species are living in the same environment. This speciation is a little more difficult to understand because the species have the occupy the same niche, however it may come down to some barriers (pre-zygotic or post-zygotic) that prevent gene flow, for example mating at different times of the day.