What exactly does Fortify Health/Magicka/Stamina increase?

I am unsure of how this works, does the effect increase both the maximum and current resource or just the maximum?

For example if I have a max health of 200 and I am currently at 100 health, I drink a potion that does Fortify Health 25 points for some amount of time. Does my current health increase to 125 along with my maximum health increasing to 225?

After some short testing, and with this UESP article, the conclusion is simply:

It increases your max. Health and your current Health for the duration of the potion. Once the potion's effects fade, you lose the same value in max. Health and current Health. That is, unless you would die just from this - you will always have at least 1 Health remaining from the potion ending:

Alchemy description: Health is increased by <mag> points for <dur> seconds.
Enchanting description: Increases your health by <mag> points.
Fortify Health boosts the target's Health by <mag> points. When the effect expires, the target loses points of health unless that would reduce the target's health to 0 or less; i.e. the target is left with at least 1 health point when the effect expires.

Note in particular the last sentence.

(The same obviously applies to the other Fortify potions as well.)