A report format changes the shape of an answer. Tabular reports list detail records. Summary reports group rows and calculate totals. Matrix reports group both rows and columns. Joined reports place separate report blocks beside one another, with a meaningful shared grouping where available.
Choose the format from the business question. A case export needs rows, while a workload comparison may need priority groups. A matrix can compare stage by region without treating every intersection as a separate report. A joined report's blocks can use different objects and filters, so inspect each block rather than assuming one filter applies everywhere. A chart title can't repair a report whose underlying population or aggregate is wrong.