Solar energy is radiation from the Sun that is capable of producing heat, causing chemical reactions, or generating electricity. There are a variety of ways in which the sun's energy can be harnessed including;
Solar Photovoltaics (PV) generates electricity. Solar panels produce DC (Direct Current) power which then needs to be inverted to AC (Alternating Current) to be used by your house and the grid, or fed directly to a battery bank in the case of an off-grid system. Solar PV is the dominant form of solar technology and most of these questions are geared towards Solar PV.
Solar Hot Water uses heat from the sun to heat up and circulate water.
Passive Solar is a building practice that utilizes the sun in passive ways. Skylights and sun tunnels are examples of passive solar design. Another example would be large south facing windows with an overhanging roof such that the house is heated by the sun when it's lower in the sky in the winter and cooled by the overhang when the sun is higher in the sky in the summer.
Why use solar energy?
Solar Energy is a renewable and plentiful resource that has few limitations. Harnessing solar energy is comparitively cheap and the energy produced can be used directly at the source as opposed to traveling great distances. The two main limitations are shading from nearby trees (nature's solar panels), and that the sun is an unreliable generator, meaning solar production/effectiveness is heavily skewed towards summer months. Net-Metering or the current Self-Generation Offset program allows a grid-tied home to produce power which feeds the house first, any excess power is then sent to the grid which is recorded by a bi-directional meter. This meter will provide credits that can be used at night and after a summer's worth of production you can offset the winter's use as well. A more detailed explanation on Net-Metering and the Self Generation Offset Program can be found here(link!) A Net-Metered Solar PV system uses the grid as an "infinite battery bank", where your excess summer production can be banked for winter.