Associations refer to Rule-Obj-Association instances which are pre-defined join conditions that the developer can use in multiple reports. Class joins are specific joins that the report would need just for that report (and not reused elsewhere).
Also, Associations are available in the report editor but class joins are not.
Click to use either actual data or simulated data in the Report Editor. Designing a report with simulated data is useful when working in a development environment with limited data. It is also useful when a report query is very expensive and you do not want to re-retrieve the data for display each time a change is made in the Report Editor. This will make editing faster.
Based on the properties that you are trying to retrieve, the report editor will show dummy data based on the property type without actually running the query and getting the data from the database. As the explanation above mentions, this is useful when the query takes too long to execute or returns too many results. Every change to the report in the editor will trigger a run of the query (modified with the changes made) to show actual data if that option is chosen.