Priority Moves
From Pokemon World Online Wiki
Priority governs which Pokémon strikes first in battle.
By default circumstances and conditions, the Pokémon with superior Speed statistics will strike first. However, if a Pokémon uses a priority move, the user of the move will strike first, notwithstanding the opponent's speed. If both Pokémon uses a priority move within the same value bracket, the Pokémon with the superior Speed will strike first.
Different priority moves may fall into different value brackets, which range from 5 to -7 in PWO. The brackets of each priority move correspond to the same value in the generation-4 games (compiled below).