Each faction has a set of primary classes (the number may vary on faction) and, depending on how you build your loadout (accessories, weapons, items, etc) for that class, there are a multitude of possible specializations. Eternal Crusade is also a massive tactical war game so not every specialization will be equal when going toe to toe with each other – you should either bring friends to make up for your weaknesses or deploy your loadout in the circumstances that favour your specialty.
Since you can change classes and loadouts whenever you deploy to the field, you’re not locked into your choice for hundreds of hours (though that’s not to say all the choices you make in your progression will be easy to reverse).
Certain classes (such as Elites) have a resource cost associated with spawning them and exceptional players can earn access to hero classes as well. A given player’s opportunity to use these characters is limited and they must be supported on the battlefield, but with their power and ability to enhance the capabilities of primary classes those around them, they can turn the tide of war. Of course, killing a hero is also an exceptional action that shall be rewarded.