All Pokemon creatures and their moves are assigned certain types. Each type has several strengths and weaknesses in both attack and defense. In battle, you should use Pokemon and moves that have a type advantage over your opponent; doing so will cause much more damage than normal.
A single-type advantage (for instance a Water attack against a Ground-type Pokemon) will net you double normal damage. The advantages also "add up", so a double-type advantage (for instance a Water attack against a Ground/Rock-type Pokemon) will net you quadruple damage. In both these cases you will see the message "It's super effective!" in-game after the attack.
Conversely, a single- and double-type disadvantage will afflict half and a quarter normal damage respectively. Here you will see the message "It's not very effective..." in-game.
Another advantage you can gain is Same Type Attack Bonus (STAB). As the name implies, this increases the power of the move if the attacking Pokemon has the same type as the move used (for example a Fire-type Pokemon using a Fire-type move). In this case the damage is 1.5 times normal. Again this is added to any other advantages, so a Water-type Pokemon using a Water-type move against a Ground/Rock-type Pokemon will bag you six times (2×2×1.5) normal damage!
Below is the full type chart, displaying the strengths and weaknesses of each type. You can also download the type chart as an image for offline use (e.g. to print out).
The above type chart has remained the same since Generation II (Gold/Silver). In Generation I (Red/Blue/Yellow), however, there were a few differences:
Psychic Pokemon fear only ghosts and bugs!However, this only affected one move: Lick. The two other Ghost moves, Confuse Ray and Night Shade, affected all Pokemon equally. It was corrected in Generation II when Ghost became super-effective against Psychic.
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