The main tanks in the game Battleheart are the knights, the two being Shepherd and The Black Knight. There appears to be no difference in the characters other than appearance. The focus of knights is to absorb damage rather than to give damage. His special abilities don't devastate enemies, but rather they focus on pulling the attention of enemies away from his allies. Knights' passive abilities focus on increasing his defense (or offense), and making enemies he's fighting easier to deal with by dropping their attack power or dropping their defense.
The other tank in the game is the monk. He has a higher DPS than the knight and can hit enemies harder than the knight, but doesn't take damage as well as the knight does.
The paladin is a unique tank who can use healing skills and high defenses to keep herself and her teammates alive.
For your team, definitely consider having a tank. They are vital to keeping the rest of your team alive.
Barbarians and rogues are the two DPS in this game. While the barbarian focuses at dealing out high damage in short spurts of time (such as an ability he receives where activating a skill will increase attack at the expense of defense), the rogue has skills that can weaken foes and cripple their defenses, such as poison knife.
Wizards and witches are magic based, wizards' skills are high, continuous damage based, with enemy knockdowns and enemy slowdowns. The witch's skills are everywhere, she has a skill that will steal health from an enemy and grant it to your ally with the lowest health, she can do crowd control with her Horror ability, or turn a difficult enemy into a frog, rendering them temporarily useless. Both are excellent choices to include on a team, dealing high damage (at the cost of a lower DPS) with very unique skills.
The ranger, Oakley, is an interesting character. At level 25 he gains a DOG that can attack enemies to deal additional damage. Oakley possesses skills with his bow and arrow that can cripple opponents rather than deal significant damage, such as a poison arrow. A ranged-attacker from the magic sub-class is usually favored over the Oakley, but he can still be a very useful character. Who wouldn't want a dog fighting on their Battleheart team?
The two healers in this game are the cleric and the bard. At the beginning of the game, the Cleric is almost always favored because of her auto-heal, but at late-game when Tier 7 items are available, the Bard is a very reliable option. Two Codex of Jabes on the Bard will give it the ability to constantly heal your party. The bard can also offer strong buffs to the character he affects, including a +50% defense or offense raise. The cleric's auto heal is very useful, and by equipping her with two Codex of Jabes she can group heal your party for every 8 seconds. The cleric can also defend herself with her Vengeful Angel skill, so both the cleric and bard are good choices.