The basic form of most Hindu and Buddhist mandalas is a square with four gates containing a circle with a center point.
In various spiritual traditions, mandalas help to create a sacred place and are used as a spiritual tool for focusing attention as an aid to meditation.
The mandala represents the cosmos metaphysically or symbolically; it is a microcosm of the universe from the human perspective.
A form of the mandala in Christianity are stained glass "rose" windows. A similar example is the dromenon on the floor of Chartres Cathedral. The dromenon represents a journey from the outer world to the inner sacred center where the Divine is found.
The Illuminations of Saint Hildegard von Bingen can also be used as mandalas.
For more information about mandalas: wikipedia
For more information about Saint Hildegard von Bingen: illuminations
A wonderful site with beautiful moving mandalas can be found here: light-weaver