Lets say I am fighting a group of 20 demons. When I right click on them I see that their health is 30 (for example), and remaining health is at 20. Then I blast the group and 3 die, but their remaining health is now 24. I find this confusing. Is it essentially saying that the total health of the demon group is 600, as in 30x20=600? And that every 30 points of damage I do effectively kills one from the group? Thanks

Yes, it is as you say. Everytimes the damage dealt to the group is higher than maximum health (or than remaining health) of the first unit in the group, one unit dies. The remaining damage is dealt to next unit, etc., until the remaining damage is lower than the maximum health.
So in your example (20hp remaining, after the attack 3 died, 24 hp remaining) the damage was 86.

Thanks for the clarification

The "remaining health" that you see is the current hit points of the first creature in the stack.
If you have 5 creatures of 30 hit points each, and 24 "remaining health", the creature health is as follows:
24 30 30 30 30
You can only ever damage the "top" creature on the stack. If you do 25 damage, it will kill the top creature, and the results would be:
0 29 30 30 30
The "remaining health" is very useful when trying to whittle down stacks of strong creatures like dragons or treants. If the "remaining health" on a dragon is 30, you could save some mana by killing it with a magic missile instead of something more powerful.