This article contains spoilers for the very end of BSG, or for fans of Caprica who have not yet seen BSG at all. Proceed with caution.
The Colonial star system consists of three stars and one 'outlier', possibly a brown dwarf or supermassive gas giant whose status as a full star is questionable. The twelve worlds circle these stars in various arrangements that allow them to remain habitable (although only borderline so, in several cases).
The Colonies vary greatly in geographic make-up and political, economic and military power. The Colonies, oddly, have only a limited selection of wildlife across the twelve worlds, most or all of it likely imported from Kobol. The combined human population of the Twelve Colonies at the time of the Fall was 50 billion, but was presumably lower (maybe a lot lower, given our population expansion) sixty years earlier at the time of Caprica.
The Twelve Worlds
Tribal name: Aries
The breadbasket of the Twelve Colonies, consisting of vast agricultural holdings. Relatively poor and uncultured compared to the other Colonies. Aerilon is located on the orbital track of the gas giant Ragnar, located at one of its Lagrange orbital points. Its capital city is Gaoth, destroyed in the Fall (an image of which becomes an iconic representation of the genocide for the survivors, and is touched for luck by combat pilots before going on missions). Gaius Baltar is Aerilon's most famous son, although he has distanced himself from his homeworld and undergone speech therapy to get rid of its distinctive accent.
Government: Democratic republic
Tribal name: Aquarius
Also called Aquarion. A frigid ocean world with very few habitable islands. This is one of the least-populated of the Twelve Colonies and has no capital city, and is barely considered an equal colony by the others. It is located at another one of Ragnar's Lagrange points. This planet is a pure democracy in which all citizens are allowed to vote on every issue. The head of government is a Prime Minister.
Tribal name: Cancer
A more Earth-like planet famed for its beaches. Its capital city is Hades, which might be the large coastal metropolis we see being nuked during the Fall in The Plan. Canceron is a balmy resort world, analogous to Hawaii or the Caribbean, although there is also a poverty-stricken underclass.
Capital: Caprica City
Government: Democratic republic
Tribal name: Capricorn
A large, Earth-like world. Possibly the largest, most populated and certainly the richest and most cultured of the Twelve Colonies (possibly the most decadent as well). Caprica is located in a conjoined orbit with the planet Gemenon, the two of them circling around a common centre of gravity as they orbit their star. Its capital city is Caprica City or Cap City, which was destroyed in the Fall. Delphi appears to be the largest surviving metropolis after the Fall.
Tribal name: Gemini
An arid, red-surfaced world which looks a bit like Mars from orbit. Gemenon is the twin planet of Caprica, the two circling a gravitational barycentre as they circle their central star. Gemenon's capital city is Oranu and its second city is Illumini, a major centre of religious study and discussion. Illumini has a huge pantheon complex where every deity in the Sacred Scrolls, even the minor ones, are worshipped. Gemenon has a wide variety of religious institutions, cults and denominations, ranging from those who take the Scrolls literally to fringe monotheist groups.
Tribal name: Leo
The planet consists of two continents and is famed for its open plains. It is a temperate world where a variety of outdoor pursuits are enjoyed. Its capital city was Luminere. The planet was noted for its alcoholic beverages, most notably Leonis Estates Sparkling Wine and Leonis Red, an energy drink. Leonis is unfortunately xenophobic and isolationist, leading to some problems with relations with other colonies.
Tribal name: Libra
The centre of legal study in the Twelve Colonies, noted for its legal firms and institutions. The planet has no official capital, but a major city is Themis. The planet has no pyramid team at the time of Caprica.
Tribal name: Pisces
Another water world, although less dominated by its seas than Aquaria. Picon is seen as a 'mini-Caprica' and stands in for that world in many television shows. The planet is colder than Caprica and is home to a solid snow tourism industry. Its capital city is Queenstown. After the First Cylon War, the planet became the base for the Colonial Fleet Headquarters, and was one of the first planets attacked by the Cylons in the Fall of the Twelve Colonies. Picon is quite blatantly the Canada to Caprica's USA.
Tribal name: Sagittarius
One of the poorest colonies, beaten down and exploited by the others for centuries. Its people are, as a result, uncooperative and surly towards the other colonies, although some have broken away from their upbringing to become successful. The planet has a different interpretation of the main religion than the other colonies, and distrust modern medicine. Sagitarron also appears to be more resentful of its integration into the federalised government of the Twelve Colonies after the war, and Tom Zarek led a terrorist campaign there designed to free the planet from its control. Its capital city is Tawa. From its economic status, Sagitarron may be one of the other borderline-habitable colonies with a hostile climate.
Tribal name: Scorpio
A hot world noted for its lush jungles and a mix of trashy resorts and exciting sports, such as paragliding. Its capital city is Celeste and its most noted resort town is Argentum Bay. The planet has a scorpion tail-like half-ring in orbit, although this is not visible in either The Plan or Razor. The Scorpian Fleet Shipyards circle the planet, and are destroyed in the Fall. Scorpia is a major centre of spacecraft construction, and its third-largest settlement is actually a shipyard in orbit (possibly the Fleet Shipyards, or their forerunner).
Tribal name: Taurus
Tauron is somewhat barren, with no native flower life, although agriculture does exist. The Tauronese are a distinctive, independently-minded people. Their colony was jointly invaded by Virgon and Leonis some 800 years ago and the Tauronese spent a long time driving them out in a guerrilla war. Ever since then Tauron has been plauged by infighting and violence, but sixty years before the Fall is now trying to go straight and build respectable industries and businesses of its own, although the crime families still exert great influence. Tauron is one of the wealthier colonies. There are strong anti-Tauron feelings on Caprica, possibly sparked by a flood of Tauron immigration to that world, which has largely vanished by the time of the Fall. Tauron's capital is Hypatia, although Tauron City is another major habitation. Tauron has a large, ringed moon and does not share its orbit with any other colony. The last land battle of the First Cylon War was fought on Tauron.
Government: Strong consitutional parliament supplanting fading monarchy
Tribal name: Virgo
Virgon is noted for its forests and is considered to be one of the wealthier colonies. The planet is ruled by a parliament which is eroding the power of an older monarchy. The planet's capital is Boskirk. During the Fall, the Colonial military launched a major counter-attack on the Cylons at Virgon which ended with the destruction of the Colonial fleet.
The Twelve Colonies were settled by the inhabitants of the planet Kobol. On Kobol the inhabitants had become divided into twelve tribes divided by ideology and religion, although broadly united in the worship of the gods, the Lords of Kobol. The twelve tribes outcast many of their criminals and religious dissidents (particularly monotheists and atheists) to form a thirteenth tribe, which eventually developed technology that allowed them to cheat death by 'downloading' and resurrecting in an artificial organic body. The other twelve tribes, scandalized, drove out the Thirteenth Tribe, which was forced into exile in deep space. Eventually they found and colonized a distant planet called Earth.
The remaining twelve tribes fell into strife and warfare, eventually rendering Kobol partially or completely uninhabitable in a massive conflageration known as the Blaze. The survivors made their way into space and were guided to the Twelve Colonies, which they settled and colonised. The initial period of colonisation, some 2,000 years prior to the beginnning of Caprica, was followed by a 'dark age' in which high technology was lost and the Colonials were reduced to using wooden sailing vessels and gunpowder-based weapons. After a time the Colonies were able to resume space travel.
Eight centuries ago, the colonies of Virgon and Leonis allied together to invade Tauron. The Tauronese liberated themselves through a lengthy guerrilla war, but by the end of this conflict violence and bloodshed had become ingrained in Tauronese culture, where a strong criminal underclass took root. A major civil war ripped Tauron apart forty years prior to the events of Caprica, but this eventually ended and Tauron became a richer, more productive member of the Twelve Colonies, providing richer colonies like Caprica with a strong immigrant workforce, although this also led to ethnic and racial tensions.
Fifty-eight years before the events of Battlestar Galactica, highly intelligent robots called 'Cylons' were invented on Caprica and pressed into service in the military and industrial sectors, where they became little more than slaves, despite having a degree of sentience and self-awareness. After several years of mistreatment, the Cylons rebelled and declared war against the Twelve Colonies. In the First Cylon War that followed, the Cylons and humanity fought one another to a standstill over the course of twelve years, before a peace agreement was reached and the Cylons agreed to leave and settle elsewhere. Forty years later, they returned and destroyed the Twelve Colonies in a surprise attack using nuclear weapons. All twelve worlds were devastated and rendered uninhabitable for several millennia to come, until radiation levels drop to a habitable level.
Less than 55,000 people survived the attack on the Colonies. Fleeing into space in a rag-tag, fugitive fleet protected by the battlestar Galactica (and later Pegasus), they eventually found and colonised another world where humanity might take root and survive.