Item Level is a statistic possessed by all equippable items (other than consumables), representing the general PvE power level of the item comparable to other items. Item level has no relationship to character level, and characters of any level can equip items of any item level, providing they meet any other requirements of the item (i.e. Player Level Requirements or Role-specific). The item levels of all a player's items are used to determine the player's combat rating.
The Item Level does not directly correlate to the item's rarity (common, uncommon, rare, epic, legendary, or artifact): an uncommon item can have a higher level than an epic item, and vice-versa. However, for attuned gear, a higher rarity would result in a slightly higher item level for the same given combat rating of the player.