Custom Report based on Last Call Results will only look at the last (or most recent) Result entered in the call history. With this type of report each contact can only be returned once.
Custom Report based on All Call Results will look at the complete call history of a contact. This means that the same contact can return multiple times in the report, once per call history.
Note: If, for example, you are running a report to see how many total calls a user has made for a time period, it would be based on an all call history report. In either case, filters can be applied to the report to limit the contacts or calls that are returned.