Cobweb

The "Web" is a new item introduced in the 1.5 minecraft update. It has not yet been found naturally, but many belives 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.

More information can be found here: http://www.minecraftforum.net/viewtopic.php?f=1020&t=295999

If you have further information to add, feel free to send Kris Eike a PM on the forums.