STAB,also known as Same-Type Attack Bonus
STAB is a 50% boost to moves which are the same type as the user.
For example,An Infernape using Mach Punch.
Since Infernape is part fighting type and mach punch is a fighting type move,there is an increase in base power from 40 to 60
Same-type attack bonus (Japanese: タイプ一致ボーナス coincident type bonus), abbreviated STAB, is a 50% boost to the power of an attack when it is the same type as one of the types of the Pokémon using the attack. This results in a 50% increase in damage (100% if the Pokémon has the Adaptability Ability).
Same-type attack bonus does not apply to moves that do not use the normal damage formula, such as Seismic Toss. Contrary to what some early strategy guides stated, Normal-type attacks can receive the same-type attack bonus.
The same-type attack bonus was introduced in Generation I and has remained unchanged since.
Source:Well it's something every Pokémon player should know and http://bulbapedia.bulbagarden.net/wiki/Same-type_attack_bonus