Internal Users are the regular employees and users of a company's network.  While internal users are able to create new workspaces and use the workspaces set up by the PBworks Admin, they only have access to content on the network that is allowed to them by the security policy of the Workspace Settings.  Because of this, content can be hidden or made read-only from an internal or external user by a PBworks Admin or a workspace Moderator.  The permission level a user has on a workspace determines what kind of changes they will be able to make on it.

Internal users have access to the Home, Dashboard, the Workspaces list, and the list of Users.

If an internal user creates a workspace, then they will have the permission level of "Moderator" on the workspace, and will have complete access to the contents inside.  However, if another user (such as another internal user or a PBworks Admin) creates a workspace, an internal user may be added with a lessened permission level, or not at all.  An internal user could be added as a Moderator of one workspace, to get complete access to one workspace, but if they could also have Writer or even less access on other workspaces.  Internal users can also be denied access to certain parts of a workspace through page and folder security.

This is why it's important not to give PBworks Admin access to all network users.  PBworks Admin access gives full access to all workspaces (with the "Moderator" permission level), and so no information on a workspace can be kept private from a user with Moderator permission on that space.

Also, on the network every workspace has it's own members list and security settings, so a user could be Moderator on one workspace and denied access to others.  This is why it's best to reserve the role of PBworks Admin to only a few users, and the default role for any employee on the network should be Internal User.