DotNetNuke is designed to host multiple portals.
Meaning you can have several web sites all pointing to one installation of DotNetNuke; each with thier own unique look, content and users.
admin:
The admin user is created during the installation of DotNetNuke, and has control over the first portal created. Any portal user can be an admin simply by assigning it the Administrators Security Role. When creating additional portals, you must specify and administrator account for each portal. The admin can change portal settings such as title, keywords, and the skin used. The admin can also add content through pages and modules, and set permission for roles to edit or view content.
host:
The host user is the SuperUser created during the installation of DotNetNuke, and has control over all portals. Additional SuperUsers can be created using the Host / SuperUsers Accounts menu option.
SuperUsers:
SuperUsers have all of the capabilities of Administrators on all sites. SuperUsers can adjust site settings such as HttpAliases, Disk Space, Site Log Retention, and Modules available.
