Hello!
The basic difference between the different classes of users is depends on what which permissions that user has. A user can only affect the files for which they have access. The root user has the capability to manage anything, also files owned by other users. Due to the fact that of this power the root user has, the root user does not have FTP or email access and can only access the server through SSH (either directly using an SSH key for the root user or indirectly by logging in as the Administrative User and then becoming the root user through commands such as su).
