Solar energy refers broadly to the technologies which convert energy from the sun into useful power, usually either electricity or steam.
The most common kind of solar energy is photovoltaic cells, which directly convert light to electricity. They traditionally have been wafers of polysilicon, but recently thin film technologies have been under development. They are less costly to produce, do not require silicon, and usually are less efficient in terms of yield of electricity produced from photo energy input. Photovoltaic systems are commonly installed on homes and businesses. In Germany, subsidies for solar energy have resulted in creation of solar farms, where people have invested in solar panels as a source of income.
Another kind of solar energy is solar thermal, where solar energy is collected and turned into heat. This method was promoted for home owners to heat water during the energy crisis of the '70s. Water circulated through roof mounted panels to heat tanks usually in the basement. Larger systems use sophisticated technology to collect sunlight in a field of mirrors and then direct those rays to a point where a fluid is heated to high temperatures. Such systems work best in desert areas, but some environmentalists find them objectionable.
Solar energy is as free as air breathing and there are many ways of capturing it with minimal capital expenditures. We can open window curtains to let in sunlight and warmth, but it only works in the southern sides of houses. The northern sides of houses are never shone on and are always in the shadow constantly. However, there is a new way of bringing sunlight and warmth into the shady northern sides of houses with means of reflecting sunlight. With use of heliostats, we can reflect sunlight and warmth back to houses. We can build heliostats with aluminium foil glued on smooth masonite boards and mounted on posts. Aluminium foil is nearly as effective as mirrors commonly used with heliostats. It has not caught on commercially or it will certainly be popular in underdeveloped nations that rely excessively on firewood for heat.
In broadest terms, photosynthesis, the process by which plants aborb solar energy and convert it to sugars and or cellulose, can be viewed as a solar energy technology. Algae farms designed to produce fuels are one example. In a sense, coal and oil are forms of stored solar energy.