Short answers, pulled from the story.
The scattered disc is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies. Its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper.
Astronomers based at Mauna Kea in Hawaii discovered the first scattered-disc object to be recognised as such in 1996. Three more were identified by the same survey in 1999, and over 200 SDOs have been identified as of 2011.
CCD-based cameras made it possible to directly produce electronic images during the 1980s, allowing surveys to capture more light than film. This resulted in over a thousand trans-Neptunian objects being detected between 1992 and 2006.
The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object, but its discoverer Michael E. Brown suggests it should be considered an inner Oort-cloud object. With a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets.
The scattered disc formed when Kuiper belt objects were scattered into eccentric and inclined orbits by gravitational interaction with Neptune and other outer planets. Computer simulations suggest this scattering took place relatively quickly during Neptune's early migration epoch.