Wort (pronounced Wert) is the hot malt liquor that beer is made from, before the yeast can be pitched (added to the Wort) it must be cooled down to around 30°C other wise the yeast will be killed. This cooling process if unaided can take hours which besides being impractical exposes the the freshly made beer to airborne bacteria and undesirable wild yeasts. For this reason the Wort is cooled down to a practical temperature using a Wort cooler or Wort chiller.
There are two common types of Wort cooler counter flow and immersion, this instructable details the build process for the doommeisters twin coil immersion Wort cooler. Immersion Wort coolers as the name suggests are immersed in the hot Wort and cold water is passed through the cooler thus bring the temperature of the Wort down.
For small batches immersion coolers are much easier to clean and sterilise than counter flow coolers, this is because they can be sterilised in the hot Wort itself by adding the cooler in for the last 15min.
There are many plans and photos on the Internet for this kind of cooler so this is not really an original design more an interpretation.