Element

Each element has a weakness and a strength.

Fire → Ice → Wind → Earth → Thunder → Water → Fire

Additionally Dark ← → Light are opposed elements.

Or, if you like lists:

Fire melts Ice

Ice blocks Wind

Wind erodes Earth

Earth grounds Thunder

Thunder shocks Water

Water douses Fire

Light illuminates Dark

Dark eclipses Light

Certain weapon skills have elemental properties, as well.

Ex: Red Lotus Blade > Fire

See also:
Weather
Ninja: Elemental Wheel
Calculating Magic Damage