TMG Reporter has three types of reports. Overview Reports, User Overview Reports and Activity Reports.
Overview Reports as the name suggests presents a nicely summarized overview of your organization's network. Overview reports have three sections. Bandwidth, Productivity and Firewall. Each section includes 'Top N' style information such as Top Users By Size, Top Sites by Size, Unacceptable Browsing By User, Top Firewall Rules and so on.
User Overview Reports are the same as Overview Reports, only they are filtered by a specific user. As such, some sections of the report are removed such as 'Top Users' and 'Top Departments'. They present a nicely summarized overview about how a specific person uses the network.
Activity Reports on the other hand are quite different. First of all, they require you to specify a filter, such as User Equal to Bob or Site Domain Equal to YouTube.com.
Activity Reports list all activity by date stamp. The 'Activity' being shown is whatever makes sense depending on the Filter. For example, an Activity Report on a single user will show a list of web browsing activity for that user throughout the day. An Activity Report on a specific Site will show you the people who browsed that website throughout the day and when.
Activity Reports show you exactly when traffic occurred and present the full URLs when clicking the summarized rows in the report.
By using the three different types of reports, and appropriately using Filters, you can easily report on what you need.