All creatures in the game are divided into four Types. These types are:
Defenders are first of the game tanks. They have the best DEF score and can reduce a lot of incouming damage. They are good against any types of units but attackers. The best (and sometimes the only) way to kill good defender is to polymorph it.
Stout is the other tank unit in game. Stouts have great HP and REG. They can withstand a lot of damage and regenerate it quickly. They are most effective in PvE since PvP combat is usually short comparing to regeneration time. A lot of stouts have good aura or abilities so you should consider having some in your army.
Ranged are usually the only units that can attack at a distance. Most of them have RNG 2 but some have RNG 3. Ranged have decent ATK but low HP and DEF.
Attackers have high SPD that allow them to act first. They also have great ATK, allowing them to defeat enemies before their turn.