The unit types in the game are Assassins (shuriken icon), Casters (gem icon), Healers (cross icon), Bruisers (axe icon) and Tanks (shield icon).
Each unit type specializes in certain aspects.
Assassins deal the most damage but aren't too resistant to damage, Healers mend your other units, Bruisers deal less damage than Asssassins but are sturdier, Tanks are sturdy but don't deal much damage, and casters are a balanced ground between assassins and bruisers with the addition of some buffs.