Rappelz/Levels/Hit points

You can see the amount of HP your character has in the green bar in the left hand top corner of the window. If you see in the green bar something like:
 * 1270/1470

it means that your character has 1270 HP of the maximum of 1470 hit points it could have.

Calculate maximum hit points
Another thing that you might notice is the steady growth of your HP whenever your character levels up. This is because of a rather simple formula, which can be used to calculate your character's maximum HP at any given level, as long as you know what your character's Vitality is.
 * Vitality * 30 + Character level * 20 = Maximum HP.

Measured values
Helps us to improve the formula above
 * I have:
 * Vitality 32
 * Character level 43
 * Job Level 31
 * Maximum HP 2000
 * Calculated HP would be: 32*30+43*20 = 960+860 = 1820

Lose hit points
Your character will lose HP when:
 * it takes damage when it is attacked by monsters.
 * it takes damage when it is attacked by other players.

Gain hit points
Your character will gain HP when:
 * it is standing, running, etc.
 * it is sitting (faster than standing),
 * it drinks a healing potion, see items.
 * it is target of a healing spell, see skills.

Death
If your character reaches 0 HP it dies. If it dies it loses EXP which is good and bad:
 * bad because in order to regain this EXP you have to kill additional monsters but
 * good because it doesn't lose JP when it dies. So after killing those additional monsters your character ends up with a better JP/EXP ratio. This makes the next battles a little easier.