Powder

Explodes if the target uses a Fire -type move this turn, damaging it for ¼ its max HP and preventing the move.

Powder 'covers the target in powder' for the rest of the turn.
A Pokémon affected by Powder will take ¼ of their maximum HP as damage whenever they use a Fire -type move and that move will fail.
Powder as increased priority meaning that it will be used before most other moves.
If another Pokémon uses a move of the same priority, the Pokémon with the higher Speed will go first as usual.
Powder does not affect that:

• are Grass -type
• have the ability Overcoat
• are wearing Safety Goggles

If a Pokémon gains one of the effects above after already being affected by Powder, it will not be cured of it.
Pokémon with the ability Magic Guard do not take damage when using a Fire -type move but will fail to use that move.
Powder does not prevent a frozen Pokémon affected by it from thawing itself out with a Fire -type move.
A berry that makes Natural Gift Fire -type will hurt the user but not consume it.
Protean can not change the user into Fire -type.
If Fire Pledge is after another Pledge move, Powder does not affect the combined move, otherwise Fire Pledge fails.
Powder is not affected by Damp.
Rain prevents the damage taken from Powder.
If an affected Pokémon uses Shell Trap, it will always receive damage.
A Fire -type dance move such as Fiery Dance will not activate Dancer.

• In Gen 6 Powder is not affected by rain .
  
• From Gen 8 onwards this move can not be selected in battle.