Crosstab reports are reports that print in a grid format. For example, the Balance Sheet - Combined report is a crosstab report, as the balance sheet accounts print in rows and the funds print in columns, with the ending balance printing as the detail information.
In order to set up a custom report in Report Writer as a crosstab report, when first creating the report, the Report Type must be specified as Crosstab (an existing report cannot be changed to a crosstab report).