Characters afflicted with a Negative Level suffers a -1 penalty to ability checks, Attack Rolls (including Combat Maneuver checks), Saving Throws and skill checks for each Negative Level that character has. On top of this, their Hit Points (both maximum and current) are reduced by five per Negative Level. If a character is afflicted with a number of Negative Levels equal to or greater than their Hit Dice (or Character Level), they will die. The cumulative effects of Energy Drain and the relatively low Hit Dice characters have (compared to their Hit Points) means even the strongest of characters can only withstand a few hits from these monsters - and their ability to competently fight back will diminish every hit.
Fortunately, creatures with Energy Drain tend not to inflict many Negative Levels per hit - typically one to three. The amount does not vary by hit, rather different creatures are capable of inflicting different amounts of Negative Levels per hit. Also note that a creature with Energy Drain can only inflict Negative Levels once per round, regardless of how many attacks it gets.
These Negative Levels are rather persistent, however, only giving the afflicted character one chance to remove them per day, and only if a Fortitude Save (which varies by monster) is made. They can be removed in a more timely fashion with Restoration spells.