Of course, ferrets have been domesticated for far less time than either cats or dogs. Quite when this first happened is unclear, although we know that the Romans bred them for catching small animals such as rabbits down burrows, and they may not have been the first to do so. Today, ferrets are often considered sufficiently different from their wild kin to be classed as a different subspecies (just as cats and dogs are). Unlike the wild forms, they are often white or pale yellow in colour, and many are true albinos, with bright red eyes. However, the coat colour can be quite variable, including tan, reddish-brown, dark brown and true black, often with markings that can be clear enough to give a 'Siamese' appearance. Although the colour variation is less than among, for example, cats, the American Ferret Association nonetheless manages to recognise a full 38 possible colour patterns for show purposes.
The wild ancestor of the ferret is the European polecat (Mustela putorius), a darkish brown animal with pale markings on the face. They inhabit forested environments, especially along the banks of streams or rivers, and are common throughout much of western and central Europe. They have fared less well in Britain, being persecuted by gamekeepers in Victorian times, and only beginning to recover after World War I. A sizeable population remains in the Welsh mountains, and has re-colonised parts of England and Scotland, although probably only after some inter-breeding with escaped domestic ferrets. On the other hand, the Scottish polecat may once have been a distinct subspecies, but has been extinct since at least 1912.
While domestic ferrets were originally used for hunting rabbits, these are not a large part of the diet of their wild kin. In most areas, their most common food appears to be frogs, especially in winter, when they can form over 80% of their diet. Of course, they also eat a lot of mice and voles, not to mention the occasional shrew, and some birds, especially where they don't live so close to rivers or ponds. One explanation for the origin of the word "polecat" is that the first half of the word is a corruption of the Old English for "foul", and both polecats and ferrets are somewhat smelly. This smell is important to the animals, giving out sexual signals, with males being particularly interested in the smell of females, and animals being able to distinguish between different individuals on the basis of their odour, and they are wary of scent marks left by animals they don't know.
Further east, we find the steppe polecat (Mustela eversmanii). Where the European species inhabits forests and watercourses, this, as its name implies, has colonised the great open grasslands and plains of eastern Europe and Asia, from the Middle East as far as Mongolia and northern China. Here, in the absence of frogs, much like those European polecats that live far from rivers, it feeds mainly on small rodents, with voles and hamsters being particularly common. In appearance, the two species look quite similar, but the steppe polecat is generally paler in colour, with a more distinct black 'mask' on its face, like that of a raccoon. Indeed, the steppe polecat looks more like a ferret than European polecats do, especially in some subtle features of the shape of the skull. While the genetics of ferrets suggest that they are, indeed, domesticated European polecats, it's entirely possible that at some point, their ancestors were cross-bred with the steppe species, and that the modern form is actually a hybrid.
At some point during the Ice Ages, steppe polecats (or possibly their immediate ancestors) crossed over the Beringian land bridge and entered North America. There, they found new grasslands, and new rodents to eat, and they prospered across the Great Plains. Isolated from their relatives, they developed into a new species, the black-footed ferret (Mustela nigripes), distinguished, as you might imagine, by the colour of its feet. Today, black-footed ferrets eat almost nothing but prairie dogs, although they will catch mice or rabbits if any are around. That can't always have been the case, since the very oldest fossils, found in Nevada, and dating back at least 750,000 years, apparently lived in an area devoid of prairie dogs, but today the fortunes of the two species seem inextricably linked. Without prairie dogs, modern black-footed ferrets just don't seem able to survive.
Which is unfortunate for the ferrets. American farmers are not very keen on having lots of prairie dogs on their land, and, as they eradicated the rodents during the twentieth century, the population of black-footed ferrets crashed. The ferrets generally stay with a particular colony of prairie dogs for much of their life, carefully not eating any more than they need to survive, but, when the rodents are gone, that close relationship becomes a major drawback. So severe was the problem that, for many years, it was thought that black-footed ferrets might well be extinct.
There were always sporadic reports of the animals, however, and, in 1981, the US Fish and Wildlife Service found what appeared to be the last surviving population, hiding out near a ranch in Wyoming. There were barely more than a hundred individuals left alive, and most of them died in 1985 after an outbreak of disease spread among the local prairie dogs, followed shortly thereafter by an outbreak of canine distemper among the ferrets themselves. By the time a captive breeding program was established, the entire world population had dropped to just eighteen individuals.
All three species of wild ferret are nocturnal, although they will move around during the day if they have to. European polecats are happy to sleep in crevices or hollow logs, and will also occupy burrows dug by other animals. In the more open plains environment of the other two species, burrows are the only realistic option, and, while they don't dig their own from scratch (why bother, when you can get prairie dogs or rabbits to do it for you?) they do often expand the ones they come across to their own liking.
They are, like many mustelids, solitary animals, only coming together to breed in the spring. They give birth to a litter of kits five to seven weeks later, depending on species, just in time for the beginning of summer. Black-footed ferrets seem to have the smallest litters, with typically only about three individuals, although its at least possible that inbreeding really isn't helping here. On the other hand, steppe polecats have an average litter size of seven, and have been known to have up to eighteen kits at a time, although, since they don't have that many teats, it's unlikely that all the members of such exceptionally large litters would survive. The young leave home after about three months, and are ready to mate as soon as their first breeding season comes around the following year.
The pelt of European polecats is considered valuable, and is sometimes sold under the name of "fitch". That of the steppe polecat is much less so, and, indeed, farmers probably gain more from keeping the polecats alive so they can eat rodent pests than they would from trapping them.
All three species can interbreed to produce fertile offspring which, has, naturally enough, raised arguments as to whether they are really different species at all; the current consensus is that they are different enough from each other for that not be an issue. They can even interbreed with their closest non-polecat relative, the European mink, although this appears to be very rare, and the male hybrids are apparently sterile, which is perhaps as well since mink are an endangered species, and wouldn't want to be bred out of existence.
[Pictures from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008.]