A hypothetical structure first described in 1959 by the physicist Freeman Dyson in a short paper published in the journal Science entitled '"Search for Artificial Stellar Sources of Infra-Red Radiation", a Dyson sphere is an artificial hollow sphere of matter around a star designed to capture nearly all of the star's radiated energy for industrial use. The original proposal by Dyson did not go into much detail about how a Dyson sphere would be constructed, focusing instead on the more fundamental issue of how an advanced civilization could expand its energy production to the maximum possible for a given solar system. Such a civilization would be classified as a Phase II civilization under the classification scheme developed by Nikolai Kardashev.
A star contained within a Dyson sphere would not be directly visible to the outside universe, but the Dyson sphere itself would radiate an equivalent amount of energy in the form of infrared light due to solar heating from within. In addition, since Dyson spheres are composed of solid matter, it would radiate a black body spectrum. Dyson proposed that astronomers search for such giant anomalous "stars" in order to detect advanced alien civilizations, but none have been recorded.
There are two basic varieties of hypothetical design for a Dyson sphere. The more realistic of the two, and the closest to Dyson's original conception, consists of a swarm of independant solar collectors orbiting in a dense formation around the star. The solar collectors could range widely in individual size and design, and possibly include space habitats for biological creatures to live in, but as a Dyson sphere they would collectively intercept almost all of the star's total output. This type of Dyson sphere is sometimes referred to as a "Dyson swarm" in order to distinguish it.
The second type of Dyson sphere is a uniform solid shell around the star, often with a layer of atmosphere and soil on the inner surface to provide an astronomically large living space for organic life forms. This form of Dyson sphere is much more popular in science fiction, but is not physically feasable for a variety of reasons. One is the immense strength that would be required for such an enormous structure, and another is the fact that the net gravitational force exerted by a uniform hollow sphere on anything inside is zero; there would be nothing holding the atmosphere to the sphere's surface, and it would fall into the sun. This type of Dyson sphere is sometimes referred to as a "Dyson shell."
See also Ringworld