The Statistics page contains useful information about the current site. Some of the fields can be confusing if you don't have inside knowledge of WebGUI. Here's an attempt to document what the numbers mean:

• Assets - any asset whose state is 'published'
• Packages - the number of assets that are marked as a package
• Templates - total templates including the default templates
• Active Sessions - (Don't know what this statistic tracks; it's often much larger than the total number of users which seems strange)
• Users - total number of users
• Groups - total number of groups; many groups are not displayed in the Group Manager; Subscription groups for the CS and Friends Network groups, to name two.