Evacuated tube collectors are much better insulated, and so are more efficient, particularly during cold or windy weather. 
An evacuated tube consists of a doubled walled glass tube, rather like a thermos flask, with a vacuum in the space between the walls.  The inner tube has a special dark coating on it.  The vacuum is a very effective insulator, eliminating conduction (where the heat travels through solid or liquid material), and convection (where the heat is carried along by a fluid).  The heat arrives, through the vacuum, as radiation, and is absorbed by the coating on the inner tube.  This causes the temperature inside the inner tube to increase.  The inner tube contains a heat pipe that is a narrow copper tube which itself has a vacuum in it, together with a small quantity of a fluid which, because of the vacuum, boils at a low temperature.  As the sun warms the tube, the liquid in the heat pipe evaporates (turns into a gas).  Because the collector is installed at an angle, the hot gas in the heat pipe rises to the top.  The top end of the heat pipe is installed in a manifold and surrounded by water, which condenses the gas back into a liquid as the gas gives up its heat to the water.  The heavier liquid then runs back down the heat pipe and the cycle recommences.  The phase changes (liquid to gas and vice versa) actually enhance the efficiency of the heat transfer considerably.

The SPS-Eco Collector

SPS’s Eco collector consists of a stainless steel frame which bolts to your roof, carrying a number of tubes which are inserted into a header manifold at the top of the frame.  Water from the hot water storage tank enters the heavily insulated manifold, and is warmed by the hot ends of the heat pipes as it passes around them.  It then exits the manifold and either enters another collector for further warming, or returns directly to the heat exchanger in the hot tank.  Gradually this cycle warms the water in the hot water tank.  The tank generally has two heat exchange coils in it, one in the lower half and one in the upper.  Using heat exchange coils in the hot water tank prevents the water from the collectors mixing with water for domestic use.  This is desirable because it allows the water flowing through the collectors to be treated with anti freeze and other inhibiting chemicals.  The collector is connected to the lower coil, because the water at the bottom of the hot water tank will be cooler than that at the top (hot water rises, cold water falls due to a difference in density), and the most efficient heat transfer takes place where the temperature difference is greatest.  The upper coil is connected to the conventional boiler for back up heating (and is sometimes replaced with an immersion heater).

Solar water heaters can be used for space heating, but it is much more usual for them to be used for domestic hot water (DHW).  The main problem with space heating applications is that heat is required at times when there is little or no sun (eg December nights).  In order to make much impact on heating requirements, larger collector arrays and large heat stores are required.