Cave Orcs are a more primitive and secluded variety of the Orcs.
Humans are a young race whose exceptionally high birthrate allowed them to spread across Eo and become one of her most influential inhabitants.
About
Dark Orcs seem to share much of their culture with regular Orcs, except the fact that they prefer to live in isolation, often in very mountainous areas or, as their name would suggest, caves, as opposed to forming more stable settlements and societies like their brethren.
This is also reflected in their tools and equipment, mainly beeing seen using simple weapons and very light, if any, armour, suggesting that their secluded lifestyle hasn't allowed them to advance to the same technological level of most other Orcs on Eo. Their magic capabilities also reflect this. While regular Orcs can become decent and certain types of magic, cave orcs are only ever seen using the most primitive spells.
Though it isn't mentioned why these orcs choose to live in such a way, it might be related to the War of Six Races, after which many Orcs chose to settle on Fiara. Many of these once isolated Orcs would eventually end up forming new tribes, such as the Sharok orcs in the nort-east of Fiara, or the Clans under the rule of the Iron Lord, south of the Gate of Kings. However, due to Fiara's vastness, it's not impossible for some groups of Orcs never to be found, resulting in them continuing their lives as cave Orcs.