Here you can view the hierarchical tree of reports and their categories. If you select a report category from the tree, you can create a New report or New category under it. Reports cannot be created directly under the root (All reports) category. Selecting a report in the tree allows you to export the report using the Export selected button or clone it by clicking Clone report. All items in the tree apart from the root can be deleted by using the Delete selected button.
Selecting an item from the tree displays its details on the right side of the screen. More information can be found in the help section of individual tabs.
More resources can be found in Developer's Guide -> Modules -> Reporting -> Overview.