A Dyson Sphere is a hollow sphere around a sun with a
circumference of 1
AU, with a hollow space inside. A Dyson Sphere of this size would have a surface area 600 million times that of the Earth. It was named after
Freeman Dyson, who proposed it as a way to capture all of the
energy radiated by a
star.
Structural elements of Dyson Spheres are interesting - the curvature of the inside of a Dyson Sphere is even less than that of the Earth. Also, there is a problem with gravity - everything wants to fall into the sun. To get gravity, you must spin it, but that makes only the equatorial regions habitable. This problem is what caused Larry Niven to cut out the top and bottom, leaving him with just a ring (ringworld).
Another way of handling the gravity problem is to have a habitable space between two Dyson Spheres, heated and given gravity from below.
There is also a way to create day and night - inside the sphere, if there were large square light-blocking materials rotating around the sun, the shadow they cast would work as night.
Dyson Spheres are popular with people who think that the earth is too small: people who like SF, Fantasy, Flat-earthers, and really anyone who wants to imagine billions of civilizations living in the same space.
In SF, the earliest reference to a Dyson Sphere was in Robert Silverberg's Across a Billion Years.
information from http://www.d.kth.se/~nv91-asa/dysonFAQ.html