The terms sound like they should describe the same thing; they are similar, but the main difference is that one is the actual way things are, and the other describes the potential for things to be different.
Variation is directly observed. It is the difference in traits in a population.
Variability refers to the potential for differences in a population. It may not be directly seen, but is useful for looking at what might be expected to happen. At the molecular level, for example, it can be expressed as a mutation rate for a particular trait.
Be careful when you use these terms. People tend to use them, when they speak about them, as the same thing, but they are not.