Item Level

Item Levels, or ilvl, is a property applied to every item to determine the strength of its magical affixes, if any, on the item. This is generally done through the process of comparing an affix's Quality Level to the item's Item Level. To name an example, an Item with an Item Level of 3 can access affixes that are in the quality level range 1-3. These generally include the weakest bonuses to stats, resists, and other things.

Diablo II/Lord of Destruction
Item Levels at first cannot be seen anywhere for vanilla Diablo II players, expansion enabled or not. You are required to use a plugin called PlugY to even see the Item Level. A key detail to keep in mind, though, is that an item's Item Level is always equal to the monster that has dropped the item- if you know the monster's level, be it through a guide or memory, you then know the Item Level of its drops. If you have gambled the item, it is then Character Level -5 through +4. If you have bought the item in or  from a vendor, it is then Character level +5. Shopping in is slightly different. You still get the clvl+5 = ilvl but there are max ilvls set by act:


 * Act I: 12
 * Act II: 20
 * Act III: 28
 * Act IV: 36
 * Act V: 45

Item levels are used mainly for verification of modifier possibilities.

Diablo III
Item Level returns in Diablo III, and once again, it is a level players cannot see at all ingame. They must view the items over in the Game Guide to know its exact level. Item Level is specific to a piece of gear, and more often than not, is not equal to the level of the monster that dropped the item in the first place. That being said, just because it is that way, does not mean affixes are rolled with the Item Level in mind. As of the pre-expansion patch, the main determining factor in an item's power is the monster level, but the hero's own level can also account for drops, to some extent; to name an example, a level 60 playing with a level 70 Crusader, although commonly thought of as more beneficial drop wise, due to the tougher monsters that the Crusader must deal with, will only be offered items that match the power of the Barbarian, while the Crusader will receive items worthy for his/her level. The reverse is not true, however; if the same Crusader were to join a game made by the same Barbarian, both would be getting drops with their level equal to that of the latter.