STAB

Stab is a key mechanic to any pokemon game. If a pokemon uses an attack of the same type as its own, they will receive a 50% damage bonus.

Examples
Rattata, a normal type, uses Tackle, a normal attack. Tackle has 50 base power, so it would look like this: Squirtle, a water type, uses Water Gun, a water attack. Water Gun has 40 Base Power, so it would look like this:
 * 50 Base Power x 150 STAB = 75 Total Power
 * 40 Base Power x 150% STAB = 60 Total Power