A capacity is kind of power, but it is not the same as an ability. An ability is a power that an agent has to perform some action. It is “a power to originate changes in the environment.” In contrast, a capacity is “a causal power…an invariable disposition to react to certain determinate changes in the environment in certain determinate ways.” (Van Inwagen, An Essay on Free Will, 10-11).
For example, Van Inwagen distinguishes between the capacity to understand and the ability to speak. The distinction lies in the fact that though someone who has the capacity to understand French has no choice over whether or not to understand French sentences when he hears them. In contrast, his ability to speak French does not entail that he utter a word. Sentences in French uttered or inscribed cause a person with the capacity to understand French to grasp the correct meanings of the sentences. But a person who can speak French is not caused to do so merely by having such an ability.
Some people have a language capacity but lack the ability to speak. There are, for example, neurological conditions that prohibit fine motor skills while leaving cognitive capacities largely intact.
