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.

Another thing that you might notice is the steady growth of your HP whenever you level up, and occassionally increase your job level. This is because of a rather simple formula, which can be used to calculate your total HP at any given level, as long as you know what your Vitality Stat is.
 * Vitatlity*30 + Level*20 = Max Hit Points.

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.