User activity is recorded for every system user. Every time when someone update his profile information, upload avatar or maybe even make password reset email request, system store that activity into database and so system administrators can see what their users are doing.
Administrators are able to see activity logs for all system users, as well as search for specific log messages only, as it is displayed on image below.
Also, if administrator is interested for activity log of particular user, simply by clicking on user's name he can view the entire log for that specific user.
Non-administrator users can see only their own activity log and search through it if they see some suspicious activity.