Favored Enemy

Favored Enemy (passive): Favored Enemy grants +2 to damage and +2 to the skills bluff, listen, and spot against a specific creature type (chosen by the player). This feat advances by +2 at levels 5 and 10. One additional enemy can be chosen at levels 5 and 10.

For example a ranger is able to select Goblinoid as his favored enemy at first level and get a +2 bonus. At fifth level he is able to select Giant and he will get a +4 bonus to both Goblinoid and Giant enemies. At tenth level he gets to select yet another class of enemy and the bonus on all of them increases to +6.

The following monster types are available to be chosen:
 * Aberration - Beholder, Rust Monster
 * Animal - Dog, Wolf
 * Construct - Inevitable, Golem, Defender
 * Dragon
 * Elemental - Fire Elemental, Earth Elemental
 * Giant - Giants, Ogre, Troll
 * Dwarf
 * Elf
 * Goblinoid - Bugbears, Hobgoblin
 * Gnoll
 * Halfling
 * Human
 * Reptilian - Kobold, Troglodyte
 * Magical Beast - Winter Wolf
 * Monstrous Humanoid - Minotaur, Gargoyle
 * Ooze
 * Chaotic Outsider
 * Evil Outsider - Fire Reaver, Flesh Render, Hell Hound, Thaarak Hound
 * Lawful Outsider
 * Undead - Ghoul, Ghast, Skeleton, Spectre, Wight, Wraith, Zombie
 * Vermin - Scorpion, Spider

For more information on Dungeons & Dragons monsters, you can visit the D20 SRD page. Note however that DDO implementation of monsters may differ from their PnP version.