Bonus HP acts exactly like regular HP. It is subject to the same damage reduction/amplification from resists. In addition, it is added to both your current and maximum health, but the additional max HP is only temporary.
At the expiration of the spell, any bonus HP you received will stay. The exception to this is if your current HP would be greater than your regular, unbuffed max HP. If this is the case, your HP will be reduced to your max HP.
For example, if you're at 400/1500 HP and you get Wild Growth'd for 300 HP, you'll be at 700/1800. If you proceed not to take any damage, you will be at about 700/1500 at the end of the 7 seconds.
However, if you're at 1500/1500 HP and you get Wild Growth'd for 300 HP, you'll be at 1800/1800. Suppose you take 200 HP of damage. This time, when the spell expires, you'll lose your extra HP. The 1600/1800 HP you have just before the ult wears off will become 1500/1500, since you cannot have more current HP than maximum HP.
One thing to note, by the way, is that all the current sources of bonus HP actually increase your maximum health (temporarily), meaning that % max HP damage sources will do slightly more damage than if you had unbuffed health.