Although recently used as a personality system, the enneagram is actually an ancient example of sacred geometry. The enneagram is a nine-poined star. Not in the same fashion as a pentagram or the Star of David wherein all the points are connected uniformly, but in a special pattern to reflect special things.

Originally brought to the attention of the western world by P. D. Ouspensky while he was detailing the teachings of G. I. Gurdjieff in his book In Search of the Miraculous, the enneagram is said to contain a huge measure of universal truth.

Most easily described as a graphical representation of the union of The Law of Three and The Law of Seven(though perhaps not like people commonly understand them) the enneagram is commonly associated with esoteric spiritual teachings such as Sufism and The Fourth Way.

I suppose it is a personality system too, but that's like saying an orange is known primarily for having counties named after it.