An ill-wishing sprite spied a witch casting a spell in the wooded lands and sought to disrupt the energies she conjured. Moving in for a closer look, it saw a brilliant shield of protective light surrounding the witch. Not at all perturbed by this new development, it moved in to cause some havoc. When it came in contact with the light, it found that it was as solid as a steel wall and barred the sprite entrance. The sprite decided that the witch had no idea of a good time and promptly left looking for an easier target.
The circle is cast in order to protect you from negative forces that might wish to upset your activities. When a circle is created, it is ill-advised to pass through it before your rite is complete, because the energies will disapate quickly when it has a hole. There is a way to pass through a circle without breaking it, and that is by creating a door. A door is a temperary opening in the circle that you may create in it by carefully pushing aside the energies, passing through, and closing it back up. You can go back inside the same way. It is really not recommended though. By creating an opening for you to move through, something else may move through as well.
Many people like to make a physical manifestation of the circle by placing beans, cornmeal, rice, or another grain in the shape of the circle around them. The energy of the circle is usually visualized as an electric blue light that surrounds the altar, so go even so far as to visualize it as a hemisphere caging in the area blocking out all negativity from entering from any angle. This isn't neccessary, unless it makes you feel better.
The technique of casting a circle can be simple or elaborate, physical or mental. The circle may require large amounts of energy depending on the type of circle cast or the potential power of a given witch, but it doesn't necessarily have to drain you. This is my favorite ritual for casting a circle that I have found, it is from Celtic Magic by D.J. Conway:
Face the East: I call upon you powers of Air to witness this rite and to gaurd this circle.
Face the South: I call upon you powers of Fire to witness this rite and to gaurd this circle.
Face the West: I call upon you powers of Water to witness this rite and to gaurd this circle.
Face the North: I call upon you powers of Earth to witness this rite and to gaurd this circle.
Stand in the middle of the Circle: This Circle is bound,With power all around, Between the Worlds, I stand, With protection at hand.
When you are done with all of your spell casting: By the power of the Ancient Gods, I bind all power within this Circle, Into this spell. So mote it be.
Face the East: Depart in peace, O'powers of Air. My thanks and blessings.
Face the South: Depart in peace, O'powers of Fire. My thanks and blessings.
Face the West: Depart in peace, O'powers of Water. My thanks and blessings.
Face the North: Depart in peace, O'powers of Earth. My thanks and blessings.
After all is said and done step away from the Circle and say: The circle is open, yet ever it remains a circle. Around and through me always flows its magical power.
You can create a circle just by taking your athame and visualizing the energy for the circle flowing through the point of the athame and draw the circle around yourself. It is just a matter of preferance