Cobweb

The "Web" is a new item introduced in the 1.5 minecraft update. It has not yet been found naturally, but it could possibly be in a later update. 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 10% of the normal walking speed. Additionally, you cannot jump while in contact with the web. Unlike water, however, falling into a web will not prevent fall damage, although it will be delayed until you make contact with the ground.

Trivia
More information can be found here: http://www.minecraftforum.net/viewtopic.php?f=1020&t=295999
 * You walk much slower than normal while walking in the web.
 * It is not possible to jump when inside a web.
 * It can be placed in and under water.
 * It is not used in any crafting recipes.
 * You can put a web on reeds.
 * Jumping from high above into a web doesn't kill you, but the moment you hit the "bottom" of the web you take fall damage.
 * You can stack webs to 64.
 * You can put webs on webs in-game.
 * Items thrown on the web will fall down after about 20 seconds
 * Animals that walk into the web will slow down