Type chart
From Pokemon World Online Wiki
All Pokémon have there own types, strengths and weaknesses. When engaging in battle, it is recommended to use a Pokémon with an advantage over your opponent; this will result in more damage on your opponent's Pokémon.
There are two types of advantages: First is a single-type advantage, wherein a Water attack against a Ground-type Pokemon, for example, would result in double damage (2x), and the second is a double-type advantage which results in a Water attack against a Ground/Rock-type Pokemon which results in quadruple damage (4x).