Negative levels: How do they work?

[Kafana]

I've seen a few abilities that bestow a number of negative levels on a character. How does that work exactly and how long does it last?

What penalties do you incur from negative levels and what do they affect?

[Bakeru]

Google is your friend! (http://www.d20srd.org/srd/specialAbilities.htm#energyDrainAndNegativeLevels)
A creature takes the following penalties for each negative level it has gained:


-1 on all skill checks and ability checks.
-1 on attack rolls and saving throws.
-5 hit points.
-1 effective level (whenever the creature’s level is used in a die roll or calculation, reduce it by one for each negative level).
If the victim casts spells, she loses access to one spell as if she had cast her highest-level, currently available spell. (If she has more than one spell at her highest level, she chooses which she loses.) In addition, when she next prepares spells or regains spell slots, she gets one less spell slot at her highest spell level.

Negative levels remain until 24 hours have passed or until they are removed with a spell, such as restoration. If a negative level is not removed before 24 hours have passed, the affected creature must attempt a Fortitude save (DC 10 + ½ draining creature’s racial HD + draining creature’s Cha modifier; the exact DC is given in the creature’s descriptive text). On a success, the negative level goes away with no harm to the creature. On a failure, the negative level goes away, but the creature’s level is also reduced by one. A separate saving throw is required for each negative level.

A character with negative levels at least equal to her current level, or drained below 1st level, is instantly slain. Depending on the creature that killed her, she may rise the next night as a monster of that kind. If not, she rises as a wight.
Basically, it gives you a nasty temporary status effect, and if you don't get rid of it within 24 hours, you loose an actual level.

Or if you're playing Pathfinder, look here. (http://www.d20pfsrd.com/gamemastering/special-abilities#TOC-Energy-Drain-and-Negative-Levels)