Heroes are a special unit that can be made from the special altar among other units. the hero has the ability to take on the characteristics of any unit you chose except ultimate units, and doubles their stats, the exception of casting and accuracy which is still increased though. Heroes also gain health and damage ove rhte amount of battles they are in by leveling, leveling is determined by how many battles a hero has been through. A hero can also gain aura to give the other units to make them stronger. Since a hero also cost double of the unit too it is often still highly valued. However it is also often a target of enemies, so do your best to protect it, each side has a way of doing so one way or another. There can only be one hero at a time.
Ultimate Units
Ultimate units unlike heroes are a specific but more powerful unit usually reflecting a certain type, of either melee, ranged, casting, or well a mix. That all depends on race of course, but depending on how well you use them one is as good as the other. You can have three ultimate units total, but rmeember they are costly.
Human: Marksman
Undead: Lich
Goblin: Robosuit Goblin
Dwarf: Battle Master