Hi Meenakshi,
Difference between UNTIL and LOOP UNTIL in a workflow is :
LOOP UNTIL is a step type of workflow
UNTIL is name of the outcome of LOOP UNTIL step type. If you click on the outcome names of LOOP UNTIL in workflow graphical view, you will be navigated to UNTIL where in you can change the outcome name.
In LOOP UNTIL you design all your business process based on your business requirement.
In UNTIL, you can only change the outcome names(reflected on workflow paths) whenever LOOP UNTIL ends or continues.
Normally we don't go with UNTIL step but change the outcomes names in LOOP UNTIL step itself. And that is the reason that most of us are not aware of this UNTIL step. 
Even I myself searched for the same just now in SWDD.
Hope this helps you.
Thanks,
Richa