Blocking

Blocking is a gameplay mechanic which allows players to block attacks.

Java Edition
Shields are used for blocking incoming attacks. one causes the player to slow to a sneaking pace, and after four ticks blockable attacks and knockback coming from the front of the player will be negated. Any effects from the attacker (e.g. Wither or fire) will not be placed on the defender. A successful block is indicated by a sound and the shield taking damage.

Blockable attacks include melee attacks, all projectiles, 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).

Legacy Console Edition
Swords are used for blocking incoming attacks. Blocking will decrease damage by 50% from arrows and explosions, by holding down the respective button for the console used (ex. Left Trigger on Xbox 360) while wielding any type of sword. Blocking does not decrease the sword's durability.

While blocking, the player moves at a speed slower than if they were sneaking.

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 1/200 blocks a second, or 2/25 pixel a second (1 Pixel every 12-13s) 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).