Blocking

Blocking is a gameplay mechanic which is exclusive to the console and pocket edition with a sword, rather in the PC edition, shields are used instead, and allows players to block attacks.

Usage
Shields are used for blocking incoming attacks. one causes the player to slow to a sneaking pace, and blockable attacks coming from the front of the player will do 33% damage from melee attacks and no damage from projectiles. Some non-damage effects of ranged attacks (such as being set on fire by a fireball) are prevented, while others (such as being set on fire by a flaming arrow) are not. Knockback from projectiles is prevented, but not from melee attacks.

Blockable attacks include mob melee attacks, arrows, fireballs, thrown items such as snowballs, damage from Thorns, and some explosions (e.g. creeper explosions or TNT ignited by another player are blockable, but not TNT ignited by yourself or by a redstone mechanism).

Video
This video is no longer accurate due to 1.9.The information in this video is useful for those playing games older than 1.9 or the console or pocket editions.

Trivia

 * Blocking allows for the slowest movements possible. This can occur when the player blocks, sneaks, and moves backward while under the effect of the slowness potion while going through cobwebs placed above still water on top of Soul Sand that is placed on top of ice. This causes the player to move at about 1/32 blocks a second,(needs recalculation after new additions) or 1/2 pixel a second (1 Pixel every 12-13s w/ new calculations) on a default 16x16 texture pack. Blocking while sneaking and with the additional events (but without slowness potion) allows the player go at roughly 0.023854961832 blocks per second (1 block every 41.92 seconds).