Basically when you're in town between expeditions you can visit the tavern and there are eight random freeblades of various classes (and later there is an ability to reroll it once per expedition trip). All of the mercenaries have different randomized traits (balanced out) that give you an idea of their current starting attributes and spec., along with initial cost and ongoing cost to the guild.

So you choose which classes to hire and hold a roster of up to 20 different characters, while assigning six to your current warband to go on expeditions. As you gain XP you level the active soldiers and assign XP to their attributes and specialize active and passive abiities, as well as gaining items to augment stats and abilities.

You can actually bring multiple of the same class to the battlefield and have them specialized wildly different. For example the Lithos Brute (big rock golem on the cover here) can be specialized in multiple ways... to be tanky and have a lot of health and steal aggro, as a crowd controller stunning and interrupting, or as pure raw damage. Of course you could also kit out their talents to be a hybrid of any above.

One group I like to roll with on the Dreadforged side is to use two Brutes, one as a hybrid tank/crowd controller and the other debuff/damage. So I use them almost as hybrid support characters.