This report will break down your sales over a specified date range based on the Categories that you have assigned to your products. This is an accrual based report so it will display revenue by default rather than what has been collected. You can run the report to show what sales have actually been collected.
You can run this report if you want to see a comparison amongst your different categories of products and how they sold in a certain time frame. If you want a more detailed report on a category, you can click [Transactions] which will open up the Product Sales report for whatever you selected.
These screenshots are of the same date range when running a Category Sales Report. The first two images show the report run for Revenue totals. The second two images show the same report run for Sales.
The Revenue version will show all fees that have been invoiced while the Sales version will only show what has been collected/paid.