The word "Mandala" is Sanskrit for "whole world" or "healing circle." Mandalas are historically used as symbols to help people meditate, and for protection and healing rituals.
Mandalas can be simple or complex circular designs which tend to draw the eye inward to the center of the Mandala design.
This is the basic structure of a Tibetian Mandala:
The mandala is often illustrated as a palace with four gates, facing the four corners of the Earth.
The mandalas in Paper Mandalas are in name only and do not represent true mandalas. Here is an excellent site that explains the meanings of true mandalas, as opposed to the geometric fun circular designs in this site.