You need to do some more reading about weaknesses then because that's a crucial part of Pokemon. Fire does extra damage to grass, grass does extra damage to water, and water does extra damage to fire. There are lots of types out there, and it doesn't always make sense, like Dragon being super effective against another Dragon.
So you have 'super-effective' moves (Fire > Grass) when the type advantage is in your Pokemon's favor, regular moves that have no type advantage (fire = fire) and then there is 'Not very effective..' (Grass < Fire) where the move does less damage than normal.
ON TOP OF THAT- Arcanine gets a STAB ( or SameTypeAttackBonus) for using a fire-move like fire-fang. So even if Bite and Fire fang had the same attack power and damage per second, fire fang does more damage because it is the same type as the Pokemon.
These advantages can make the difference between beating Venasaur, and just barely losing. You'll notice a CP 2000 Venasaur takes more damage from fire fang than a CP2000 Blastoise or Charizard- so knowing type advantages will really help.
Check the "Combat" Section of this website, I found this there.
STAB (1.25)
Type Advantage (1.25)
Type Disadvantage (0.8)
Critical (1.5)
