Each race of the Kor Union has it's own role. The Goblin infantries are great at scouting the map and sickening the enemy heroes with disease, while the Orc infantry can take a lot of damage and are resistant to missiles. However the Orcs do not have too much anti-air support and can not repair their buildings.
Another important unit for the Kor Union is the Orcish Wolf Rider. The Orc Riders have great speed and, when equipped with Torches, they become useful at razing enemy structures and mines. Wolf Riders are great at establishing map control.
Later in game, you gain access to the powerful Ogres which, in great numbers, can become quite a threat to the enemy. Also you gain access to the Kobold Snipers, long range high damage missile infantry.
However a disadvantage of the Kobold Snipers is that they produce from your Orchold level 2. You may need to build a Torture Tower and train Dwarf Slaves as your builders so you don't need to interrupt the production.
The Goblin Chieftains can summon Goblins on the spot. They might not seem as useful but with their help your army can easily outnumber your enemy's army. Another great Goblin unit is the Goblin Shaman. The Shamans have both powerful offensive spells, as well as powerful support spells. Every army is strong when backed by one of these.
When you want to assault an enemy base and want to use siege, you have to options, the Trolls and the Goblin Shooters. The Goblin Shooters are long ranged siege weapons that shoot goblins. Yes, goblins. These goblins have a high chance to get up and join your side in battle. The trolls might not have as much range, but they make up for it by being though and dealing more damage. They also regenerate twice as fast than any other troops which prove to be quite useful.