Defog lowers the target's evasion one stage, even if the target is behind a substitute. It bypasses accuracy checks to always hit, unless the target is in the semi-invulnerable turn of a move such as Dig or Fly.
Defog clears away any fog on the field, as well as Light Screen, Reflect, Safeguard, Mist, Spikes, Toxic Spikes and Stealth Rock on the target's side of the field.
A strong wind blows away the target's barriers such as Reflect or Light Screen. This also lowers the target's evasiveness.