During the campaigns you'll get some Heroes. Those characters are mostly like the normal troops, they can be upgraded with special abilities and they can carry artefacts. The difference is that characters can have more of all this, characters can carry up to three artefacts and be upgraded up to the 10th level. Normal troops can only carry one artefact and reach level five.
You'll get some important heroes and some characters, that are less important. The important ones, mostly the first character that you'll get in the campaign, mustn't die. If they die the game is over and you'll have to restart the level with a saved game.
The first and most important character in the human campaign is Derrick Pfeil and he mustn't die during the whole campaign. Derrick Pfeil is a mercenary, King Victor hired him to defence the southern borders. Derrick has to defend the Human Lands against the Orcs and protect the cities of Verson and Sylent. He also has the order to take some orcish fortifications by storm and push the Orcs back into their lands. He is a warrior who uses swords and he has some tactical skills.
Master Brennok is the only mage you'll get in the Human campaign. In some levels you have to protect him against the Orcs and so he mustn't die. In all other levels he can die but he will not be available for the rest of that level then. Master Brennok is from the town of Sylent and a real mage. He gets a new spell at every level-up. You can also give him one or two attack/defence skills. He joins your troops at an early stage of the Human campaign.
Father Dexter is a high priest, he joins you in the fourth level of the Human campaign. He can die and is not available then for the rest of that level. Father Dexter gets some strong healing and resurrection skills at his level-ups. He fights with a sword and seemed to me a bit stronger than all other heroes. He is the one who knows about the artefact in Verson which you have to protect in a later level.
Ugraum is the boss of the orcish horde. He's the one you have to lead through the orcish campaign without dying. Ugraum wants to combine the Goblins and Orcs to a Horde who is able to kill all the Humans. He is a strong warrior and can get also some tactical skills.
The next hero you'll get during the Orc campaign is Besargar, the chief of the Berserks, he'll join your troops in the third level. He is a warrior like Ugraum and gets also some tactical skills during the level-ups.
Ash-Hoon is joining your troops at a later level, he's a Goblinhexer with many practical spells. But be careful, at one point in the game he'll betray you and you have to fight against him.
Sho-Doon is a Goblinhexer too and you'll get him a bit earlier than Ash-Hoon. At a later point in the game you have to fight against him, so he can help you to catch Ash-Hoon. At last you have to kill Sho-Doon too.
Teya Tael is also called Teya Dragonspeaker by the elves. She is the Alliance hero that mustn't die during the whole Alliance campaign. She's an elven mage with many healing spells, but she has also some attacking spells.
I don't know, how this dwarf is named in English, but Dwarvenrock is the best translation I think. Dwarvenrock is a dwarf and warrior, many fighting skills he can call his own once you've pushed him up a bit. However, he will help Teya to fight against all evil things on her way through the Alliance campaign. You'll get him in the first level together with some dwarven troops.