Impacted is direct, so a Service Failure will impact a Service, but that Impacted Service Failure will Affect your application. The application isn't rendered dead, but a functionality of it might be affected. Hence the two terms of Impacted Vs affected.
to expand a little bit on Chads response, impacted entities will represent a short list (also shown in the UI) of entities impacted by the problem, focused on the level of Services, Applications and RUM.
While affected entities will show all entities that are affected by the problem. To get a better look at this pull the problem via the API v2 and have a look at these fields: 'impactedEntities' and 'affectedEntities'