'Immortal' (often, simply, 'immort') refers to players who have reached the highest attainable levels in the game. Here, anyone of level 102 or higher is an 'immortal.' Immortals are capable of exceeding the usual game rules, in more drastic and pervasive ways as levels increase; examples range from teleportation and invisibility to mortal sight to creation of objects, removal of a player's privileges (channels, ability to move, or even ability to log in), and ability to promote players to immortality.
Since immortals can wreak havoc with game balance, they are (ideally) a carefully-chosen and policed group. An absolute rule of immortal behavior is that no immortal can aid or assist any player without explicit permission from the higher ranks; in other words, please don't ask an immortal to help you kill a mob, gather EQ, etc, or you'll likely be either politely or rudely refused, or referred to this file.
The duties of immortals, and even the very reasons for their existence, are topics of continual and heated debate in various MUD forums, and are fraught with misconceptions, poor examples, and outrageous hypocrisies. :)
For a detailed description of each god type help and their name Here's a list of all the Gods in Dragonlance: