The Burn status effect is one of the more useful status effects. Its reduction of opponents' HP over time as well the detrimental effect to their physical attack power is very useful.
Burns reduce the afflicted Pokémon's HP by 12.5% at the end of every turn. They also reduce that Pokémon's physcial attack power by 50% (1/2).
Aside from being inflicted from a large variety of moves the Burn status effect can also be inflicted by throwing or holding a Flame Orb. To throw the item use the move Fling. You must be holding the item to throw it... It is suggested the item be thrown the first turn the user is in battle, otherwise Flame Orb will inflict a burn on the user.
Fire type Pokémon are immune to the burn status effect.
Pokémon with the ability Water Veil are immune to the burn status effect. If the weather is Strong Sunlight Pokémon with the ability Leaf Guard will not burned. Likewise, if the weather is Heavy Rain Pokémon with the ability Hydration will be cured of this effect.
Further, Magic Guard Pokémon will lose no HP from being burned (but will still have their physical attack power halved) and Pokémon with Heat Proof will instead lose only 6.25% of their maximum HP instead of the normal 12.5%. A Pokémon with the Guts ability will lose 12.5% of their HP but instead of their physical attack power being reduced by 50% (1/2) their physical attack power is boosted 150% (x1.5).