A pokemon can be one (or two) of a bunch of different types like "bug" or "rock" or "ghost," each of which comes with a particular set of types it's vulnerable to (2x damage taken) and another set of types it's resistant to (0.5x or 0 damage taken). So no one pokemon has an advantage over all the others, and there's no 2-3-4-...-J-Q-K-A strength ordering for types.

You can also freely substitute among your team of 6, though subbing replaces your attack for that turn. If you anticipate the opponent using an attack of a particular type, you can sub in something resistant to that attack.

In single-player/story mode, you never encounter an opponent at the maximum level, and the NPCs tend not to build type-diversified teams, so the player gets a huge advantage in most battles by just grinding a couple pokemon of the right type.