Cobweb

The "Web" is a new item introduced in the 1.5 minecraft update. It has not yet been found naturally, but many believe it will in a later update. Current thoughts are that spiders will nest in it, or shoot the web at you to slow you down.

The web is designed to slow you down, and limit your movement abilities. When you are "inside" the web, you will move in a speed equal to about 10% of the normal walking speed. So, your speed is 10% instead of the normal 100%. You can currently place the web on reeds, which might be a bug, but is not confirmed as a bug.

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