Generally, spells and rituals require the casting of a circle. The circle is used to invoke a spirit, to welcome spiritual powers and unite with them, to meet the Goddess and the God. It symbolizes unity, wholeness, the cycle of seasons, the wheel of time, and is normally 9 feet in diameter. Movement should be clockways to create, and counterclockways to negate or unwind something. Some covens prefer to anoint their Third Eyes with a blessed oil and ask for protection.

Some circles aren't even circles at all, they are spheres. But ultimately, the majick circle defines the ritual area, encompasses the altar, all participants and their personal power. It also shuts out the negative energies from the majick being worked, and I find it also helps to shut out the distractions.

Here are some examples of casting a circle, and some other relevant information:

How To Cast A Circle
Casting A Coven Circle
The Easy Way To Cast A Circle
How A Circle is Used