The Ba gua are eight diagrams used in Taoist cosmology to represent a range of interrelated concepts. Each consists of three lines, each either “broken” or “unbroken,” representing a yin line or a yang line, respectively. Due to their tripartite structure, they are often referred to as “trigrams” in English. These eight diagrams represent Heaven, Wood, Water, Mountain, Earth, Thunder, Fire and Lake.
What is it used for, or what is it?
Simply put: A Ba Gua is one of the tools used by a feng shui master, along with a Luo Pan (feng shui compass) to help determine preferred locations, in the home or office, for certain functions; such as wealth, health, fame, love and more.

A bit more info on: http://en.wikipedia.org/wiki/Bagua_(concept)
Here is a nice diagram explaining how it relates to your different body parts, colors and life situations:

Li
Fame / Rank / Reputation
Eye
Red
Kuen
Marriage / Mother
Abdomen Organs
Red / Pink / White
Dwei
Children / Descendant / Offspring
Mouth
White
Chyan
Helpful People / Travel
Head
White / Gray / Black
Kan
Career / Ancestral
Ear
Black
Gen
Knowledge / Self Cultivation
Hands
Black / Blue / Green
Jen
Family / Health
Foot
Blue / Green
Hsun
Wealth
Hips
Blue / Purple / Read
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