Joined reports versus report type joins
Key Takeaway
A report type join happens inside a custom report type, where you link a primary object to related objects. A joined report is a report format with up to five blocks, and each block has its own report type and its own filters.
Salesforce gives you two different ways to combine data from more than one object in reports. A report type join happens inside a custom report type: you link a primary object to related objects, and every report built on that report type can use their fields together. A joined report places separate blocks side by side in one report, and each block is based on its own report type.
How a report type join works
A custom report type defines which objects and fields a report can use. You pick a primary object and then add related objects, so a report built on it can show fields from the primary object and its related objects together.
You can also expand the data available to report users by including fields from objects with lookup relationships.
You can't change the primary object after you save the custom report type, so pick it carefully at the start.