Difference between dialog user and service user

Hi SRM gurus,

1) What is the difference between a dialog user and a service user?

2) When we give an user called RFCUSER in the sm59 logon as a service user we have our backend connected to the srm frontend? If I give any other user also with his/her pwd it gets connected.

Why I have to use the service user in this case

What are the implications when we use the dialog user in this case?

We are working on Classic scenario SRM 5.0.

Best Regards,

Thamizh.