Report types serve as the basis for report creation. They determine the object on which a report must be made, the fields available in the report, the columns displayed by default and the standard filters available.
Salesforce made two report type templates available to users: standard report types (available by default) and custom report types (adjustable based on your own needs).
Note: When creating personalised report types, it is possible to include up to 3 associated objects and to configure the fields (delete, rename, categorise, default columns).
Warning: deleting a type of custom report will delete the reports created using it.