There often isn't any difference. However, you cannot use 'can' to talk about a future ability. Instead, you must use 'be able to' (i.e. will be able to, going to be able to, etc.)
There is a difference in meaning and usage when the past forms are used. 'Could' is used to refer to general ability in the past whereas 'was/were able to' would be used to talk about something that was poss