My answer is copied from this very useful webpage, with illustrations, which shows you exactly what a slinky pipe heat exchanger is. Technically, as it says here, it is a curtate cycloid. It is related in a way to the slinky toy Terry mentioned, which was shaped like a spring. A slinky is better than a straight pipe because it involves much less digging: it "concentrates the heat transfer surface into a smaller volume, requiring less land area and shorter trenching":
http://c03.apogee.net/contentplayer/?coursetype=geo&utilityi...

Apart from the coils, the slinky includes straight sections at either end to connect it to the heat pump manifold. The slinky pipe can be laid with the coils either horizontal or vertical. The coils can overlap, but in your case they don't: the diameter of each coil (each circular loop of pipe) is 900 mm, and the pitch (the distance between the centre of one coil and centre of the next) is 1250 mm, which is more, so the coils here don't overlap.

This document on slinky installation is also very useful and well illustrated:
http://www.kensaengineering.com/Library/Fact-sheets/slinkyin...

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Note added at 1 hr (2012-11-08 14:00:32 GMT)
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The overlapping type of slinky is called a "compact slinky" (pitch smaller than diameter) and the non-overlapping type that you have is called an "extended slinky" (pitch greater than diameter). See the first source cited above.