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Triangles are a geometrically stable shape but also suggest direction and sense of purpose. The triangle has been adopted into various symbol systems, for example, a pink (usually inverted) triangle represents gay pride.
The circle appears in every culture as an archetypal form representative of the eternal whole. With no ending or beginning, it revolves in an eternal cycle and is linked to the sun-disk and the attendant concepts of the yearly cycle, the moon, and the wheel. In some symbol systems it also represents the universe.
The square represents the formal, mathematical, scientific order of the universe. The square represents earthbound matter, and correspondingly, with its two sides delineating a two-dimensional surface, may symbolize the earth or ground, or a field, especially in eastern pictograms. In Buddhist symbolism the relationship the square within the circle represents the relationship of the human and the divine.
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Taken from: http://www.godweb.org/shapes.htm
In Christian tradition even simple shapes are symbolically potent. A circle, for example, being without end, is a symbol of the divine; a triangle refers to the Trinity, a line, like the horizon, can mark out the bounday between heaven and earth, or between the sacred and the secular.