Cobweb

The Web is a new item introduced in Beta 1.5. It was previously found in the terrain.png file, but had no block ID assigned to it. It is not generated or crafted in any way. Current thoughts are that spiders will nest in it, shoot the web at you to slow you down, or drop it as loot.

Moving into a web limits your movement abilities. When you are "inside" the web, you will move at a speed equal to about 15% of the normal walking speed. Additionally, jumping will vibrate the player, akin to jumping in a 2 block high space. Unlike water, however, falling into a web will not prevent fall damage, although it will be delayed until you make contact with the ground.

It currently has no use, placing it in a crafting table has no effect, nor is it craftable from String. In update Beta 1.6.3 It has been named "cobweb"

History
Notch originally planned that spiders will produce webs.

Trivia

 * Web blocks can be used as air pockets in the same manner as Sugar Cane.
 * Webs can be placed on webs.
 * Items thrown on the web are also slowed and will fall down after about 20 seconds.
 * A block of Sand or Gravel which falls or is shot into a web, will slow down, then be destroyed and become a resource after about 4 seconds. Primed TNT seems to be not affected by this phenomenon, as it doesn't disappear and will eventually explode destroying the web.
 * Animals that walk into the web will slow down, even spiders.
 * Minecarts treat it as a solid block, like glass mixed with wool.
 * Players are unaffected by webs when in a minecart, except when they are in the way of the cart. This allows creation of a mob-slowing tunnel that can be ridden through with ease, making safe rooms even safer.
 * Signs and paintings can be placed on webs.
 * Ghasts will not see players in or behind webs.
 * Webs do not stop chests from being opened.
 * Jumping from as little as one block above a web, onto it, will induce half of a heart in fall damage.
 * Arrows shot do not go through webs or slow down.
 * Any kind of mechanism (excluding Dispensers and Note Blocks) can not be placed on top of webs.
 * When a player flies over a web and drops into the web, it will allow them to hold the jump key to fly straight up.
 * Players are unable to jump while in a web.
 * Webs can be placed on Sugar Cane and in Water.
 * When falling into a web, if a player gets into a minecart or boat before hitting the ground, they will not take the fall damage until they exit the cart/boat.
 * Webs have proven to be useful particularly in water-filled moats 3 blocks in depth. Adding a layer of web to the bottom of the moat will render any mob (or players) stuck. It is possible to get out (granted you will lose health from a near drowning experience) but most mobs will drown. This is a particularly efficient method of repelling creepers from hiding around the corner of your house entrance.
 * Webs seem to not be able to make the terrain biome recognizable, confirmed on SMP, while riding a minecart falling into a web with water below.
 * For example, the biome was a forest, which was supposed to make it rain, instead there was snow.
 * In SMP, the particles given off when breaking a web may slow down whilst falling, as they appear before the web block disappears. This also seems to cause them not to fly outwards, but just drop directly downwards after the web disappears. This phenomenon only seems to happen if there is a considerable amount of lag on the server.
 * Placing webs on ice will increase the slowing effect of the web.
 * Webs can be used as "quicksand", but it cannot suffocate the one who enters it.
 * Webs are called "Cobwebs" in the Statistics screen.