in terms of magic work, a circle is the area of sacred space in which magic work is done. Circles can be "called"/invoked/set up in many different ways, but the common standard is to call each of the elements (with calling the watchtowers being the most used there), then bind the four energies together. A "physical" circle may be drawn in the air, from point to point, with a hand, wand, staff, or athame, although this is not necessary as the circle can be completed (as well as started) mentally. Once a circle is in place, it should not be broken until you release the elements. if people need to enter or exit the circle, a "doorway" should be cut (this is usually done with an athame.) After the spell or ritual is finished, the circle should be released as it was called, first unbinding the weave, then releasing and thanking each element in turn.