Scottish folds are a breed of domestic cat, with the defining characteristic that their ears are folded forward and down. Outside of that, Scottish folds can be short or long haired, and have any colouration and pattern, although points (like a Siamese) are rare, and not accepted by some show associations. They are typically medium-sized cats with a generally rounded look: round head and face and stocky body.
The ear-fold is a random mutation that first appeared 1961 in Tayside, Scotland. All Scottish folds trace their ancestry to Susie, a white mouser with folded ears. The kittens are born with straight ears, and around 3 weeks old some of them will fold over. The folded-ear mutation is an incomplete dominant; not all kittens in a litter will display the characteristic fold. It's linked to a heavy bone structure, and sometimes related skeletal abnormalities such as stiff or fused joints. Breeding two folded-ear cats is very much not recommended: it will raise the number of folded-ear kittens, but also significantly raises the chances of joint problems. As a result, straight-eared cats from folded parents are desirable breeding stock, since they have the potential to produce folded kittens with a relatively low risk of health problems. (My fold, Max, had a couple fused vertebrae in his tail: it wasn't, as far as I can tell, painful, and didn't slow him down any, but you could feel the stiffness if you ran your hand down his spine.)
Generally speaking, Scottish folds are quiet, affectionate, laid-back cats. They love company, and prefer to be around their humans whenever possible. They deal well with other pets, feline and otherwise: when she was still actively breeding them, my mother-in-law routinely had half a dozen or more loose in the house, along with whatever dogs, cats, turtles, or other critters my sister-in-law had adopted at the time.