I disagree with some comments above that the elements and their corresponding weaknesses are depicted by 2 triangular sets. I feel that it should be depicted by a circular relationship instead.
My version and explanation:
Fire is weak to Water (Fire is extinguished by Water)
Water is weak to Lightning (Water is "shocked" by Lightning)
Lightning is weak to Earth (Lightning is "earthed" by Earth)
Earth is weak to Wind (Earth gets blown away by Wind)
Wind is weak to Ice (Wind cannot pass through solid blocks of Ice)
Ice is weak to Fire (Ice is melted by Fire)
Fire < Water < Lightning < Earth < Wind < Ice < Fire
This forms a circular elemental cycle where each elements opposes one element and complements two other elements.
I was searching for a chart but could only find this http://wiki.ffxiclopedia.org/wiki/Element
Note that my idea was not taken from that source but I found it coincidental that it matches what I am trying to illustrate.
Using the chart above, we can interpret it this way (using Lightning for example):
Lightning is weak to Earth and strong against Water. It is neutral to Ice and complements Fire and Wind. (When Lightning strikes, Fire may be born. Likewise, strong wind causes friction and static energy which in turn leads to Lightning discharge.) Edited, Mar 22nd 2011 1:11am by lvh