Pokémon Omega Ruby and Alpha Sapphire/Basics of Battling

Pokémon battles make up the majority of the games. Whether it's against a wild Pokémon, an ordinary Trainer, or the Elite Four, the mechanics remain the same, and you must remember them if you want to progress.

Types
Every Pokémon has either one or two types. As of Omega Ruby and Alpha Sapphire, there are eighteen different types. Typing is extremely important to remember, as a Pokémon's type determines which moves it is weak to and which it can resist, which defines most battles. If a type is weak to another type--for example, a Fire-type Pokémon is weak against Water--it will take 2x the regular amount of damage from an attack of the type it is weak too, and the attack is described as being "super effective". If a type is resistant to another type--such as a Grass attack hitting a Bug-type Pokémon--then the damage taken will be reduced to 0.5x. Some types are not affected at all by moves from other types: Ghost-types cannot be hit by Normal moves, and vice-versa.

If a Pokémon has a secondary typing, this is also factored into its resistances and weaknesses. If one of the types has a weakness that the second type is resistant to, they will cancel the other out; a Fire-type is weak against Ground, while a Bug type is resistant to it, and a Fire/Bug type will be hit for normal damage. If two types share a weakness or a resistance, this will stack. A Pokémon that is doubly weak to a type--say, a Water/Flying type being hit by an Electric attack--it will take 4x the normal amount of damage from moves of that type. A Pokémon that is doubly resistant to a type will take only 0.25x the amount of damage from those moves. If Pokémon has a typing that cannot be affected by another type, this will remain, even if its secondary typing is weak to that type; a Ground/Flying type cannot be hit by Electric moves, even though the Flying type is weak to them.

For instance, the normal power of the move Water Gun is 40. If that were to be used against a Fire-type, the power would double to 80. If Water Gun were to be used against a Grass-type, its power would be halved to 20. If Water Gun were used against a Fire/Dragon type, the power will be 40. If Water Gun were to be used against a Fire/Ground type, its power would be quadrupled to 160! If Water Gun were to be used against a Water/Dragon type, its power would be quartered to 10. If Tackle, a Normal-type move, were to be used against a Ghost-type, the move will not affect the Ghost.

See this Attack chart for a full chart on the resistances and weaknesses of types. It may look daunting, but eventually you will grow to memorize it.