Silk is a natural fibre produced by invertebrates which can be woven into ropes and cloths with a high strength to mass ratio, pleasant texture and good insulating properties. In the period before antibiotics became commonplace silk was also valued for its tendency to leave fewer potentially infectious fibres in a wound.
Commercial silk production is normally farmed from moth cocoons - this is a low volume, fairly labour intensive process and, combined with its desirable properties has made silk historically valuable. For several centuries silk production was a proprietary technology of the Chinese Empire and a closely guarded industrial secret - a secret which nonetheless eventually leaked out into the rest of Asia and thence to the world in general. Although in the modern era silk faces competition from man-made fibres in many roles it remains popular for many forms of high quality clothing ranging for business suits to lingerie.
Silk is also spun by other arthropod species - commonly bees, ants and wasps, but rarely if ever on a commercially exploitable basis. Many species of spiders also produce silk - albiet normally loaded with adhesive for hunting purposes: spider silk is particularly well known for its strength and has been the subject of much imitation in its own right for artificial fibres such as aramid.
Silk secreted by molluscs - normally by partially sessile shellfish -is known as byssus or sea-silk and has traditionally been rarer, finer and more expensive than true silk. Historically it was said to come from "sea sheep" or similar animals, but this seems to have been either myth or deliberate misinformation.
As a high quality cloth, silk has frequently been the subject of various sumptuary laws down the ages - to varying degrees of effect - and is, curiously, forbidden to men by Islam - although no-one is entirely sure why.