They may be generated randomly in narrow spaces. Two will always appear below a Giant Spider, and sometimes they will spawn on the ceiling, with a Spinner Spider inside. Temporary webs can be fired by Giant Spiders or by a Web Gun.
When the Spelunker is inside a web, their movement speed will be reduced and they will have a very low jump height, although they will be able to jump in midair.
Enemies and Damsels can also be caught in webs. They will be completely unable to move until the web is broken. Spiders, Giant Spiders, and the Ghost are exempt from this and can move through webs freely.
It is also noteworthy that Shopkeepers can still turn and shoot when caught in a web.
Movement within the web will cause it to degrade and eventually fall apart, and it's often a good idea to remove webs pre-emptively in case they become problematic in the future. A web will also degrade if there is a physics object or enemy inside of it, although it will not disappear entirely until the player intervenes. Webs can also be destroyed instantly by the Machete or Crysknife, or burned through by a Torch.
Convenient webs can be used to simulate the effects of Bomb paste. Due to the guaranteed webs underneath a Giant Spider, this trick can be used to kill them easily. Make sure, however, not to hit the spider itself with the bomb; only hit the webs.
Due to that fact that the Spelunker can jump midair while in a web, the Web Gun can be used to cross gaps or climb walls.