List R/3 User Types
Dialog users are used for individual user. Check for expired/initial passwords.  Possible to change your own password. Check for multiple dialog logon 
A Service user - Only user administrators can change the password.  No check for expired/initial passwords. Multiple logon permitted 
System users are not capable of interaction and are used to perform certain system activities, such as background processing, ALE, Workflow, and so on. 
A Reference user is, like a System user, a general, non-personally related, user. Additional authorizations can be assigned within the system using a reference user. A reference user for additional rights can be assigned for every user in the Roles tab.