Photo voltaic conversion uses the property of some materials that cells are made of which absorb solar radiation and this creates a difference in voltage between different areas of the photo voltaic cell. Though the difference in voltage between bands in a single cell is quite small, it can be made sufficiently large by joining several photo voltaic cells in series. The resulting voltage source can them be used as a source of energy. Photo voltaic cells that are available today are are not very efficient and there is also the problem that they can be used only during the day when sunlight is present. At night the solar power plant does not produce any power.
Solar thermal systems use the heat from the sun to run normal turbines. The solar energy is first concentrated using a large number of flat or parabolic mirrors to a small area. Doing this is sufficient to raise the temperature to very high levels. The heat is then used just as heat from fossil fuels like coal or gas would be used to boil water to run turbines and produce electricity. The heat generated can also be used for other industrial purposes, water treatment, etc. At night, when there is no sunlight, normal fuels can be used.