Shears

Shears are a tool used to obtain wool from sheep, collect Red Mushrooms from Mooshrooms, getting string from cobweb, destroying wool blocks more efficiently, and harvesting placeable leaf, vine, dead bush, and tall grass blocks.

Uses
Shears are the most efficient way to acquire wool from sheep (each yielding 1 - 3 blocks when sheared). Sheep can be painted with dye before shearing to efficiently produce colored wool.

Shears can be used to obtain placeable leaf blocks. Using it on any leaf block will yield a single, placable leaf block, as opposed to saplings that are dropped upon destroying the leaf block without the shears. Once placed, the leaf blocks will not decay when they are too far away from a wood block. In a similar manner to harvesting leaf blocks, shears can be used to harvest Grass, Ferns, Dead Bushes and Vines.

They can also be used to destroy cobwebs in half a second to yield a piece of string, and harvest Red Mushrooms from a Mooshroom, which turns it back into a normal Cow.

To shear a sheep in Pocket Edition, you must touch and hold down on the screen, similar to breaking a block. If you do not, you will simply hurt the sheep and eventually kill it (8 hits, 2 hits with stone sword).

History
In Beta 1.7, shears were introduced into the game. They were first mentioned by Jeb on Twitter The tweet revealing Jeb's work on shears was written in Swedish (Jag jobbar på shears nu) which translates to "I'm working on shears now". Shears were originally planned as a way to defuse TNT, but Jeb dropped this shortly after deciding TNT would only detonate with redstone or fire.

Before Beta 1.7, sheep would drop 3 wool blocks at most when hit, but shears can harvest up to 4 wool blocks per sheep.

Since Beta 1.8 they can be used to harvest Fern, Vines and Long Grass.

As of Beta 1.9. they can harvest Red Mushrooms from a Mooshroom and turn it back into a normal Cow.

As of 11w49a, sheep only drop up to 3 wool blocks when sheared, but they now also regrow wool when eating grass, which is part of a plan to have animals stay in the game longer.

Since 12w06a one can use shears to harvest Dead Bushes.

Trivia

 * Shears are the only tool except the sword that has a specific right-click action (shearing sheep and mooshrooms) and left-click action (collecting leaves, vines, string from cobwebs, dead bushes, and tall grass).
 * Also shears are the only tool that are used to break blocks that only have one version (made with iron ingots); as opposed to pickaxes, hoes, axes and shovels that can be made out of wooden planks, cobblestone, iron ingots, gold ingots and diamonds.
 * Hitting a wool block with shears will break it quickly, but not damage the shears themselves.
 * If leaves are sheared with shears, they will not drop saplings, unlike using a tool with a Silk Touch enchantment on leaves, which will occasionally drop saplings.