| 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 entangles 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 either break the web or slip out of it. Some types of enemies are immune to nets (jellies, non-corporeal spirits, crimson imps, giant spores, anything giant-sized or larger, etc.), and thus are not affected by webs. Also, all spiders (but not other arachnids like scorpions) are immune to webs; this includes you, if you're in Spider Form. The difference between a web and a net trap is that a web does not vanish after triggering, and since it extends from the ground all the way to the ceiling, it can affect flying creatures.
Webs can be found in certain rare Dungeon vaults and the Crypt, and they are 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 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. Just don't expect a web to slow down a rampaging emperor scorpion or dragon.