Druidry doesn't use the standard four element system that other Neo-Pagan traditions use. The common four element system of Earth, Air, Fire and Water derives from the Eastern Occult schools and the Greek mystic/philosopher Pythagoras is usually credited with its introduction to the West. The Celts and Druids used a system known as "Duille" (pronounced Dool-Yuh or dwee- luh)and means leaf in Old Irish Gaelic. The Celts obviously had and understanding of Microcosm and macrocosm and that the external is mirrored by the internal. Those familiar with the Wiccan system may find this elemental system a little difficult to grasp at first, but given time it will make sense. The Celts used this system from birth, it was part of their mind-set, and once we make it part of our own it becomes easier. The Celts probably didn't use the system the same way that we do, what we know for sure is it was used by healers to help assess a patients health. Upon meditation with the system you should see it's practicable application over a broad base. What makes this system unique is that it is composed of nine elements which the Celts believed were necessary for life. Take any of these elements away and life would cease (at least as we have become accustomed to). This is a three fold system, it includes an internal, external and a "directional" point of view. And there being nine elements in each of the three categories, there are sacred numbers within sacred numbers.
If you are familiar with the four element system, you've probably noticed that there is not mention of fire in this system. The reason for this is that the ancient Celts viewed fire as a personification of deity. Fire is alive, and was deemed worthy of all the respect that other creatures were given. To understand this look at it this way, fire is born and it dies, it feeds and grows and in it's own way it reproduces itself just like any other being. When we think of fire remember that fire is the embodiment of creativity and inspirational.