From Philip Proefrock:
Evacuated tube heaters are another method of collecting solar energy. Rather than running the water through a long circuitous course, each tube is a separate heat collector. It is made of a clear glass cylinder which allows sunlight to pass through, and a central heat collector tube. The evacuated tube insulates the collector element, which makes it more efficient in colder environments. The collector itself is typically filled with an antifreeze mix rather than just water. The top of the tube has a heat exchange element which is prevents contamination of the water being heated. The tubes are collected together in an array, with a manifold across the tops, containing the heat exchangers.
(Read the rest over at Green Building Elements.)