| A spider's web. The spider who left it must have been huge, as it seems to be made for trapping [player's species] rather than flies.
Any creature caught in it will send out telltale vibrations, alerting and attracting spiders nearby.
A web is a trap which ensnares anything which steps onto it as though it were hit by a throwing net. The victim becomes entangled and cannot move or attack until they slip out of the web. Some types of enemies are immune to nets (jellies, non-corporeal spirits, etc.), and thus are also not affected by webs. Additionally, all spiders (but not other arachnids like scorpions) are immune to webs; this includes you, if you're in Spider Form. The main difference between a web and a net is that webs are not destroyed after their victims escape.
Webs can be found in certain rare Dungeon vaults and the Crypt, but they are mostly found in huge numbers all over the Spider's Nest. Any monsters with the web sense property (i.e. spiders) are alerted when something blunders into a nearby web.
More so than most traps, webs can be used to your advantage against careless enemies. Characters, especially Short Blades users, can deal preposterous damage to an enemy before it is able to escape.
Prior to 0.23, giant-sized monsters were immune to webs and would destroy webs as they walked through them.