cPanel is Web.Net's domain management system. The cPanel management tool can be accessed via the web at http://yourdomain.com/cpanel (copy and paste that into your browser, but replace "yourdomain.com" with your actual domain name. You will be prompted for your user name and password. This was given to you when you signed up for your services and are the same credentials you use to log into your FTP account.
Through cPanel, you can manage your domain email accounts, create or manage your databases (available only with Soapbox, Megaphone or Campaign hosting packages), and most other aspects of your website.