The wiki does have a bit of information a lot of it depends on what gear you have.
Agility, Balanced, and Defensive heroes can be used as tank units.
Agility, Balanced, and Attack heroes can be used as offensive units.
Agility and Balanced are the two main split ones that it depends on how you gear them and skill them.
Giving an agility hero the dodge skills can help them be a tank dodging incoming attacks where their base defense is lacking as long as you have good tanking gear on them.
Tank units are: Flyers, Infantry, and Cavalry. Flyers are the best type of dodge units for tanks.
Offensive units are: Spell casters and Archers.
Spell casters are slow but have high damage.
Archers are fast but have lower damage.
You can balance out a spell caster by using an agility hero to make them be fast to be able to hit before others. While you can use an attack hero on an archer and make it still go fast and hit harder.
It depends mainly on your gear and how you work everything together to get the best results. There's a lot of variables going around that makes working it out perfectly to suit which heroes you have, which you can combine with which, and what luck you had with the tavern on heroes.
Hope this helps.