The World of Pern - 1. History of Pern

Pern is a planet orbiting the star Rukbat. It is earth-like and when first surveyed by human beings, seems like an ideal place to colonise (little do they know...)
It's well off the beaten track as far as planets go, so a group of humans, fleeing the aftermath of a brutal space war and hoping to recover the idyllic agrarian dream of their long-ago ancestors on good old Earth, send three spaceships full of colonists to settle there.
Part of their plan is to keep only as much technology as they need to lead a comfortable life, so on arrival, anything useful from the spaceships is cannibalised, effectively cutting off any hope of returning to the rest of civilisation.
Unfortunately for the colonists, just eight years later, many of them are wiped out by a deadly rain of mycorrhizoid spores (1) from another planet with an elliptical orbit that is drawing closer to Pern. They discover that every two hundred years(2) or so, this reddish coloured planet(3) will pass close enough to drop its lethal cargo and that the incursions will last for approximately fifty years(4) at a time.
The spores attack all organic matter and mindlessly devour it until there is nothing left, at which point they die. They can't eat through rock, metal or certain plastics so these act as a useful shield. They also drown in sufficient quantities of water (the sea, lakes, or very heavy rain) and can be destroyed by freezing or burning.

Being a resourceful bunch, the colonists don't immediately throw their hands up in despair (well, some of them do, but if you want to find out about that, read 'Dragonsdawn' by Anne McCaffrey). As there is no way of fleeing the planet, they come up with a plan for survival, the first being that houses need to be made of material impervious to Thread, the second being to find a way of destroying it before it reaches the ground and eats all the crops and livestock out in the open. Initially, they still have a few small shuttles and other flying craft, but there is a finite amount of fuel for these and insufficient resources to manufacture more, so they need to come up with a long-term solution less reliant on technology.

One of the native species discovered during the first eight years are small flying creatures that become known as 'dragonets' or 'fire lizards' from their resemblance to the mythical beasts of old Earth. They hatch from eggs and the hatchlings become emotionally attached to the first creature that feeds them when the shells crack open (a bit like ducks). The main difference is that the dragonets are empathic and share emotions both with each other and with the particular humans who first feed them and thus become their friends for life. They also have the ability to teleport(5) from one place to another and to belch flame after chewing a phosphine-bearing rock(6) that is widely available around the surface of the planet.

Among the colonists is a renowned geneticist, Kitti Ping. After much experimentation, she manages to tinker with fire lizard DNA to produce much larger creatures - dragons - who have the same innate abilities of their smaller cousins and act as the first line of defence against the deadly spores - burning them in mid-air before they can harm any people, crops or livestock on the ground.
When dragons hatch, they are starving, having used up all the nutrients in their egg. They telepathically bond(7) with young humans who have the requisite degree of empathy and who first feed them when they emerge. This bond is for life. If the human dies, the dragon will suicide by teleporting into nothingness. If the dragon dies, the human feels as if part of themself has been ripped away and often (although not always) chooses to die as well.
Kitti's first eggs, engineered in the lab, hatch successfully and as they grow, prove to be able to carry their human partners, teleport, breathe fire and most importantly, to breed. However, this also causes other problems...

I'll go into the specifics of fire lizard and dragon mating later, but suffice to say, when the larger and telepathically enhanced dragons fly to mate, they broadcast their passions so strongly that any humans in the vicinity are overcome by feelings of lust. This leads to people sleeping with others who aren't necessarily their regular partners, with resulting jealousy, fights and accusations. This becomes one of the reasons that dragonriders start to live separately from the rest of the population(8) (the other being that dragons take up a lot of space and that the amount of time given over to their care means that their riders don't have the time to grow their own food). During the years when Thread falls, they are too busy protecting the planet to do much else anyway.

The initial crisis results in technology being abandoned much faster than the colonists probably wanted or imagined. They are also beset by further problems as the proximity of the Red Star causes earthquakes and the eruption of volcanoes that were previously dormant. The settlers chose to land on the more fertile and warmer southern continent, but due to the eruptions and the need to shelter from Thread, they move north, where there are many more natural caves and mountains that can be used to house people (and dragons).

The division between those who ride dragons and those who don't starts almost right away. The general population live in settlements known as Holds, while the dragons, riders and their support workers base themselves in Weyrs. As time progresses, the initial agrarian (and idealistic) dream turns into a semi-feudal society. A huge amount of technological and medical knowledge is lost. Knowledge is disseminated through oral, rather than written history. For many people, there is no hope of social advancement, other than being found acceptable by dragonriders(9) and to have the chance of bonding with a dragon. Most people believe that the notion that their ancestors came from another world is only a legend.

Glossary of Terms
(1) These become known as Thread, from their thin, ribbon-like appearance as they fall from the sky. I've always imagined them as being a sort of lethal noodle.

(2) The Pernese start off calling their planet's full circuit of Rukbat a year, but after a while they begin to term this period of time a Turn (always capitalised). They still refer to months, although a week is later referred to (mostly) as a sevenday.

(3) The rogue planet becomes known as the Red Star due to its colour (and presumably because of the association of red with danger).

(4) The fifty or so Turns during which the Red Star is close enough to drop its deadly spores on Pern becomes known as a Pass. The two hundred-odd Turns between Passes are known as an Interval.

(5) The ability to teleport becomes known as going between (always italicised). Between is a black, cold nothingness in which there is no air to breathe and a human cannot hear, see or feel anything. In between, no one can hear you scream. It takes around eight seconds to teleport between places.

(6) This rock naturally ends up being called firestone. Dragons and fire lizards ingest firestone into their second 'stomach' in which the natural acids react to make a gas which ignites when they breathe it out.

(7) The unbreakable bond between dragon and rider is known as Impression. Dragons must Impress when they hatch, or they die. At every Hatching in a Weyr, there will always be more candidates than eggs, to ensure the hatchlings have plenty of choice. On rare occasions, if a young dragon can't find the person they want on the Sands, they will try to find their match among the spectators.

(8) The dragonriders, their dragons and the support workers live in communities known as Weyrs. These are extinct volcanic craters, usually situated among mountains and a long way from the rest of the population.

(9) For anyone who lives outside the Weyrs, the process of being discovered as having the minimum empathic requirement to Impress a dragon is called being Searched. Riders go out on Search whenever there is a clutch of eggs on the Hatching Sands.