Photothermal energy is harnessing the power of the sun in solar collectors, then converting it into electricity.  There are three different types of collectors that are used to collect it:  low, medium, or high.

Low collectors are simply flat panels (on a roof, for example) that collect low voltage heat for things like swimming pools.  Medium collectors are also flat, but they have larger panels so they can capture a little more energy to be channeled into things like hot water tanks.  High collectors, on the other hand, use large mirrors or lenses that concentrate the power of the sun to such a degree that they can help send electricity to generators or power plants.  They are used by many industrial plants to offset, supplement, or completely supply their power needs.