Wicca honours not only the deities, but also the elemental forces of nature. Every time we cast our circles, the Elements are a part of what we invoke.
The concept of four natural elements -- Earth, Air, Fire, and Water -- may easily be traced back to Wicca's ceremonial magical roots, but it is far more widespread than that in Western philosophy, and far more ancient. It is taken for granted in classical Greek philosophy. The attribution of various aspects of human existence to these elements is equally ancient. Other cultures have created similar systems; the Celts, for example, honoured Land, Sky, and Sea; the Chinese system includes both metal and wood as elements.
Although we, in the age of modern physics, no longer believe that all matter is composed of the four classical elements, we may still see them reflected in modern science as energy states: solid, liquid, gas, and plasma.
To attribute the elements to the four compass directions is also an ancient practice bridging many cultures. Ninety degrees is about the range of view of most human beings; four people standing together in a square and facing outwards can look in all directions.
Some systems include additional directions. These may include some combination of above, below, behind, before, leftward, rightward (these four forming another, more subjective ninety degree grid), and within or centre.
The directions may also symbolically be matched to the seasons of the year, times of day -- anything which can be mapped onto a circle.
Different cultures and groups have different ways of mapping the Elements onto the compass directions. The system most commonly found in the Craft is based in Western Europe, where the Atlantic Ocean was to the west, tropical heat to the south, barren rock, forests, and icy mountains to the north, and dry continental winds came from the east. But you may find local variations.
You will certainly find variations within the Craft, upon which element is honoured by which colour of quarter candle. The system you find here is the commonest, but it's not universal. If in doubt, anywhere in the northern hemisphere, look for the red candle. It probably marks Fire, in the South. Better yet, if in doubt, ask.
Once you are aware of the symbolism of the four classical Elements, you'll find subtle expressions of them in many places. Christian and Jewish houses of worship are full of them: the four archangels, the four gospels, the architectural detail of the four wings of cathedrals. Look also at secular public buildings, at traditional formal gardens, at the suits of ordinary playing cards. Often symbols are retained long after their meanings have been forgotten by those who use them.
Take a full week -- or up to two, if you wish -- for each of the elemental exercises below. Give equal time to each of them. You need not take them in the order listed; some people prefer to do opposed pairs (i.e. air/water, fire/earth) one after the other. But do make sure you've done all four of the individual Elements before moving on to the exercise on Centre.
Be prepared for one or more of the Elements to manifest itself in your own life, in unusual or powerful ways, while you're working intensively with it. Usually these encounters work out for the better; if not (if, for example, you're studying Fire and feel a sudden overwhelming lust for your spouse's boss) thank the Element politely (in circle, if it seems appropriate) for its attention, and then ask it to back off. To let your life be ruled for too long by any one Element is to throw yourself out of balance; know when to call it quits with these exercises.
You may wish to precede and/or follow this series of exercises with a guided meditation on the Elemental Gates.