Primary: The fastest attack, spammable, deals half damage to shields.
Special: Slower than primary, deals double damage to shields. Normally not too important.
Heavy: Slowest out of the three but hits the hardest, recommended to use on ultras. Since its the slowest it gets compensated by a 50% dmg boost. Deals half damage to shields also.
As you level you also deal more dmg by an additional: Your level / 200. This scaling however does not effect PEs.
Critical hits are always 4 times your normal damage in each slot and is calculated after scaling.
Your super is always based on your primary damage, only the damage super does 10 * your primary dmg, the others do the same dmg as a normal primary attack.