User Activity reports provide simple breakdown of which users have been accessing the administration interface
Bandwidth usage refers to the amount of data transfer your site utilizes which is automatically renewed each month (or billing period) by your provider.
The Site Statistics is a graphical reflection of the site snap shot which can be read as performance over time that can also be viewed in a range of day, week, month or year.
The site snapshot under the Reporting module allows a quick view on your site performance in the key areas that apply to your business.