The Hitchhiker's Guide to the Galaxy is a science fiction comedy series created by Douglas Adams. Originally a radio comedy broadcast on BBC Radio 4 in 1978, it was later adapted to other formats, and over several years it gradually became an international multi-media phenomenon. Adaptations have included stage shows, a "trilogy" of five books published between 1979 and 1992, a sixth novel penned by Eoin Colfer in 2009, a 1981 TV series, a 1984 computer game, and three series of three-part comic book adaptations of the first three novels published by DC Comics between 1993 and 1996. There were also two series of towels, produced by Beer-Davies, that are considered by some fans to be an "official version" of The Hitchhiker's Guide to the Galaxy, as they include text from the first novel. A Hollywood-funded film version, produced and filmed in the UK, was released in April 2005, and radio adaptations of the third, fourth, and fifth novels were broadcast from 2004 to 2005.
The title is the name of a fictional, eccentric, electronic travel guide, The Hitchhiker's Guide to the Galaxy, prominently featured in the series.
After mice, the second most intelligent species on Earth were the dolphins. "Although they had long known that Earth was about to be destroyed, their attempts to communicate this knowledge to humanity were misinterpreted as attempts to jump through hoops for bits of fish. They left the Earth just prior to its destruction, but left humanity one last message, a triple backwards somersault through a hoop whilst whistling the Star Spangled Banner, when, in fact, the message was this: So Long, and Thanks for All the Fish. The line was also the title of the fourth book in the series, and appears in that book as a message inscribed on crystal bowls left as parting gifts from the dolphins to the human race.
Source: Wikipedia, "The Hitchhiker's Guide to the Galaxy", available under the CC-BY-SA License.