There are three types of damages in Mobile Legends:
- Physical Damage
- Magical Damage
- True Damage
Physical damage is dealt using physical attack (similarly for magical damage). Both these damages can be countered using their corresponding defenses. True damage can not be countered using any form of defense or item, but true damage can be blocked using shields.
The damage you deal to an enemy depends on his ability to counter your (PHYS/MAG) attack using his defense, but this defense can in turn be reduced using penetration. Penetration decreases the enemy unit's defense stat, so you can deal more damage.
Spell vamp gives you lifesteal on physical damage dealt using your skills. Some heroes have passives for spell vamp (Alucard, Ruby) while others can get spell vamp using items like Bloodlust Axe. Spell vamp is very useful for heroes that primarily rely on their skills to deal damage for eg. Hayabuza, Martis, Lapu-Lapu etc.