The Activities Report can be used to show user activity on a weekly, monthly or yearly basis.  When the report loads, it defaults to the current month and year and all users are selected.  As pointed out by the purple arrow in the figure below, the Generate Report button is disabled when the report is first loaded.  In order to generate this report, you must first choose which activity or activities you would like the report to return, as shown in the figure below.  


Once you have select the activity(ies), the Generate Report button is now enabled.



Once the report has been generated you can then group the report by a number of different fields and/or filter the report by any text or numerical value.