Use of SAP System User

Hi All

I have a small querry in SAP there are different type of Users
one of them is System User I want to know why the system user is created and if created what is use of them and how they are associated with the Dialogue User ,As we all know Dialogue user can perform all work in Sap by logging in the sap server , so why the system user is created and how they work in sap server as well as are they associated with Dialogue user
bye