Hate

The word "aggro" is traditionally used in MMORPGs to mean the system by which enemy NPCs decide which Player Character they are attacking. Used as a verb, "aggro" means to entice monsters to attack you in preference to others. DDO's aggro system is undocumented and has only been slightly explained by the developers, but as far as we know it works as follows:

Each monster has a hate list of each known enemy, paired with the degree to which it hates that character. The list is constantly sorted by hate, and whoever is at the top of the list is chased and attacked. When a monster first detects a character he is added to the bottom of the list (multiple detections at the same time add closer characters higher on the list). Any character who drops below 1 hp is removed from the list, although a monster with a long-lasting combat routine may continue a few more attacks.

The only way to add hate to your entry is to damage the monster. Each point of damage creates one point of hate, although certain special effects may adjust that: The exact way those factors stack together is not known, although it is apparently they stack somehow (either additively or multiplicatively)
 * The Subtle Backstabbing Rogue enhancement reduces hate by 10-40% per melee hit, when active.
 * The Brutal Fighting Warforged enhancement increases hate by 10-40% per melee hit, when active.
 * The Divine Righteousness enhancement for Paladins increases hate by 100% per hit, temporarily.
 * Wizards and Sorcerers can get Subtle Spellcasting enhancements to reduce hate by 10-40%.
 * Attacks from the Treason weapon do 20% less hate.
 * Attacks from the Maul of Malice weapon do 20% more hate.
 * Wearing the Wretched Twilight reduces hate from spells and ranged attacks by 15%.

In addition, two Charisma skills can temporarily override the hate sorting: Note that if there is nobody else in the hate list, those skills have no effect. (On rare occasions Diplomacy had worked to push monsters away from attacking a solo character, but that bug has presumably been fixed by now)
 * A successful Intimidate check on a monster will bring you to the top of the hate list for 6 seconds.
 * A successful Diplomacy check will bring you to the bottom of the hate list for 6 seconds.

Although most monsters use the aggro system as described, there are some exceptions.
 * The Warforged Titan's aggro is either partly or completely randomized.
 * Stormreaver clears his hate list each time he casts Reverse Gravity.
 * Flesh Golems largely randomize their hate list.