Note: All of my information on Pern is taken from what I gleaned by reading Anne McCaffrey's _Dragonriders of Pern_ series, including but not limited to Dragonsdawn, Moreta: Dragonlady of Pern, Dragonflight, Dragonquest, and The White Dragon, Dragonsong, Dragonsinger, and Dragondrums, All the Weyrs of Pern, The Masterharper of Pern, The Dolphins of Pern, and various short stories in McCaffrey's anthologies. Some of what I say might be contradicted by one or more of the books, but that's ok, because one or more of the books are contradicted by each other, and I go with the version I like best.
The most basic idea of Pern is that dragons are the good guys. When the first Pern books were being written, Western dragons had a pretty bad rap. McCaffrey's self-stated goal was to write a scenario wherein they were not evil. Since her books took off dragons have been portrayed in a more favorable light, but when she started writing the books in the early 1960s, this was a radical idea.
Pern is a planet with a rich history of several thousand years (known as "turns") and a complex socio-political structure. That being said, I'll be touching almost exclusively on the dragon aspect of the world. Those whose interest is piqued by this summary are encouraged to read the books. Later on I'll give a booklist with two different recommended reading orders.
The aforementioned socio-political structure is a result of a menace called "thread". Thread is actually a mindless life form which devours almost everything it touches. There are three main ways to thwart it: it cannot eat through stone, fire turns it to ash, and water drowns it. The original settlers, who were technologically advanced, genetically manipulated some native fauna into fire-breathing dragons, which combat thread before it hits the ground.
The genetically enhanced dragons are not only larger than their housecat-sized native cousins (large enough to be ridden by humans), but also form a telepathic as well as an empathic bond with their riders, called "impression". As the dragonets hatch they find one person with whom they bond. The bond is one of true love and faithfulness. Riders discover, upon impression, another half of them they had never realized was missing. A dragon will never betray his or her rider, and vice-versa. Even if they argue, the rider and dragon do so with an undercurrent of love for one another which prevents any true asperity or hostility. With only one exception, a dragon whose rider has died immediately kills him or herself (and in the one exception, the dragon suicides as soon as a specific condition is met). Riders who lose dragons often do not survive their dragons' deaths, and those who do end up empty husks who are, at best, half-alive.
Pernese dragons, unlike Western dragons, do not hoard treasure, nor are they scaly; they have soft, supple hides. They have to chew a phosphorous rock called firestone in order to breathe fire. They are unable, due to genetic instructions programmed into them by their creators, to harm humans or other dragons, with highly specific and rare exceptions. Nor do they speak. They do make sounds, like their genetic cousins, things like warbles and trumpets and trills. They hear and understand human speech, but they usually rely on their riders to relay the message if they need to "talk". There are a few exceptions, but those are the foci of several of the stories, so I'll leave it at that.
Dragons are color-coded -- gold and green are girls; blue, bronze, and brown are boys. They are also stratified by size. Golds are the largest, bronzes next, browns middle, blues next, and greens are the smallest. Throughout most of the books, women only bond with golds, and golds only with women, while men bond with blues, bronzes, browns, and greens. In some of the very early and very late books (Pern-chronology) women also impress to greens.
Golds, who can't breathe fire, are the only fertile females. When it comes time to make dragon babies, the gold dragon "rises" and is chased by bronzes (and, in rare instances, browns). The winner mates with her. A few months later she lays a bunch (clutch) of eggs, which then incubate on the geothermally heated sands. While the eggs incubate (harden), dragons are sent out to "search" for potential riders for the soon-to-be-born dragonets. No one knows how they do it, but the dragons somehow sense a young person has the proper traits and the young person is invited to be at the hatching. These young people are called "candidates".
Dragons name themselves upon impression, and their new riders are the first to hear the dragonets' names. Their names all (with no exceptions) end in the letters "th". Most dragon names are not real words or Earth-style names, though it's not unheard of for them to be; two prominent dragons in the series are named Path and Ruth. The dragons take about a turn and a half to mature (golds mature at about three turns). During that time they and their riders are called "Weyrlings" and spend the time learning to live with each other and, more importantly, learning to fight thread. Even in times when thread isn't falling (called "Intervals") they learn to fight so they can pass the knowledge to their successors.
As noted before, there is more to Pern than just dragons, but what I've written so far is what people need to know in order to understand the MUSH on which Andy and I met -- tomorrow I'll be tying both together. If you're interested in learning more about the world of Pern, the original books are written by Anne McCaffrey and are usually filed under science fiction or fantasy.
The order in which to read the books is a highly inflammatory subject. If you want to read in order of publication, start with Dragonflight, Dragonquest, and The White Dragon, followed by the Harper Hall trilogy (Dragonsong, Dragonsinger, and Dragondrums). For Pern chronology, start with Dragonsdawn, Chronicles of Pern: First Fall, Dragonseye, Moreta: Dragonlady of Pern and its companion book Nerilka's Story (the latter is a retelling of Moreta as seen through the eyes of a minor character in Moreta), then start on Dragonflight and continue from there.