Webs are the source of silk, and appear randomly in fortress areas that are near chasm tiles or underground rivers. The web itself looks like a light-grey gem, and is most often labeled as being from cave spiders, but the presence of the actual vermin is not necessary for the web to be formed on a given spot (though cave spiders will appear in the general biome where the webs are found). Similar webs can be found in a biome inhabited by giant cave spiders; an actual GCS will be present if these are found.
Webs will be collected automatically by dwarves with the weaving labor enabled, but not until the fortress has a loom. A collect web task will be triggered at the loom as long as webs are on the map and in reach. This can be comfortably deactivated with - - . An important option since webs often occur in dangerous places you might want to secure first. The weaver will then drop the collected thread in the loom and, depending on the loom setting ( - - ) and availability of a stockpile, either leave it there, immediately weave it to cloth, or put it on a stockpile. Other ways to prevent collecting are preventing any dwarf from using the loom (e.g. with the menu), deactivating the weaving labor on all dwarves or removing all looms. (Lack of stockpiles alone will not prevent this!) An easier alternative is just to suspend the collect webs task in each loom. ( - ) If the webs are outside, or it requires an above-ground journey to path to them, changing rders to "Dwarves stay ndoors" will also prevent collecting.
Giant cave spiders leave similar, individual webs behind, but also shoot long strings of dangerous entangling webs at targets prior to attacking - this behaviour can be used to create giant cave spider silk farms. It is not advised to allow weavers to randomly gather webs on maps where giant cave spiders might be lurking nearby.
Webs are delicate, and can be destroyed by contact with any creature (except a dwarf that is gathering that web), or by any other rough treatment such as irrigating, etc. It also appears that if open to the outside, rain will destroy the webs remarkably fast
Vermin that come into contact with a web will immediately turn into remains. For sanity reasons, cave spiders themselves are immune to this effect.