To affect something is to have any effect on it, to make a difference (remember the difference between affect and effect).
To have an impact (on) something means to have a strong effect on it (of course, an impact also occurs when something comes forcibly into contact with something else – in a collision or wedged and crushing like an impacted wisdom tooth).
So everything that has an impact on something has an effect on it, affects it, but not everything that affects something else has a strong enough effect to be an impact.
We also have the tricky issue of the phrasal verb impact on – “low interest rates have impacted on saving”. People tend not to like new phrasal verbs, and this one is seen as business jargon. If you’re tempted to use “have impacted on”, try “have had an impact on” instead: you know you’re safe with that one