Powder moves with priority and covers the target in a powder that lasts for the rest of the current turn only. It damages the target if they use a Fire type move in that turn, and prevents the Fire move from working. http://pokemondb.net/move/powder
But how much does it damage the target? I've been wondering this for a while!
25% of targets HP.
Source
I see people leaving answers in the comments. .-.
Powder will do damage like confusion, except the power of the powder explosion relies on the fire type move's Base power. While confusion relies on how high your attack Stat is and how low your defence is.
I just edumakated you on Confusion and powder.