We mentioned this in passing, but it's time to explain this in full. One thing that makes Pokémon stand out is the intricate Type Match-Up system, which is basically a complicated version of Rock, Papers, Scissors.
Each Pokémon has one or two types; which reflect what kind of Pokémon they are. A fish Pokémon will be a Water-type; while a plant Pokémon will be a Grass-type. Similarly, each move has a type that reflects it. A move called Flamethrower is going to be Fire-type, for instance.
How does it affect battling though? Well, like in real life, some things 'beat' others. Water douses fire and in the same way, Water-type moves used on a Fire-type Pokémon deal more damage. Fire-type moves also deal less damage to Water-Type Pokemon because of this.
Not all types are strong or weak against all other types; but there are many that work in this way. Some types can't even damage Pokémon of certain types, such as Poison-Type moves when used on a Steel-Type Pokémon. Taking proper advantage of type match-ups is what separates the best from the rest.
Our Handy Type Match-Up Table
The below table is structured so that when you want to check it; simply look at the left column first to match what move-type you're using and then look right to match the Pokemon-type you're facing.
Example: If we were up against a Fire-type Pokemon, then from the table we can see that using another Fire-ype against it would be a waste of time as it does only half the damage. However; if you had a Pokemon with Water , Ground or Rock-type moves, then you'd be dealing 2x the damage levels.
When an opposing Pokémon is dual-typed, things get slightly more complicated. What happens is that the damage multipliers for both types are multiplied together.
Example A : If we were up against Virizion, a Grass/Fighting Pokémon, then a Fire-type move would do double damage, because Fire is super effective against Grass and does normal damage to Fighting. Similarly, a Dark-type move would do half damage because it's rubbish against Fighting and normal against Grass.
The kicker is that Flying-type moves do quadruple damage, because they're strong against both Grass and Fighting. Ouch!
Example B : What about Scrafty, the Fighting/Dark Pokémon? Flying-type moves do double damage, as they're strong against Fighting and normal against Dark. Dark-type moves should be avoided as they do a quarter damage, being both rubbish against Fighting and Dark.
Don't bother with Psychic-type moves either, as despite being strong against Fighting, they do no damage to Dark, which means they do no damage to our crafty Scrafty.