The Cobweb is a block with a gray/white texture that slows any players (not monsters) that collide with it to about 25% of normal speed. Cobwebs are found in all environments above and below ground. They are generally found in small clumps of 1-15 tiles, but it's also possible to find very large clusters of them. The easiest way to find them is to search underground until you find a cave filled with spiders, as these usually have over 100 cobwebs within, which rapidly grow back.
Cobwebs can be destroyed with any tool, if the player runs into them, or by coming into contact with Water or Lava.