how to use Test Users for Functional Testing

For Testers.. For Testers.. For Testers..

When you do testing for applications "Web Applications", How can you define the test Users just for testing purpose and NOT Real Active users so that will enable you to login to the system and do all kinds of testing e.g. Functional Testing ? specially when you have many different Users Category (e.g. Normal Users, Super Users, super admin ...etc. ) for one system and you to go through all these categories to make sure the test Passes all Cases ?

This is really matters for me, so i would be grateful if you share it with your colleagues to help me out with this.

Thank you all.