Durability

From the Minecraft Beta 1.7.3 Wiki

Durability is how much use an item can take before it breaks.

Breaking

Each use costs an item durability. The item breaks once the total cost passes its durability, not when it reaches it.

An item costing 1 per use lasts one use more than its durability. A wooden pickaxe has a durability of 59 and breaks on the 60th block.

An item costing more per use lasts its durability divided by the cost, rounded down, plus one.

The use that breaks an item still takes effect. The block still breaks, the hit still lands, and armour still reduces the damage that breaks it.

A broken item disappears. Nothing repairs a worn one.

What costs durability

Item Cost
Pickaxe, axe, shovel 1 per block broken, 2 per hit
Sword 1 per hit, 2 per block broken
Hoe 1 per block tilled
Shears 1 per leaf block or cobweb broken, 1 per sheep sheared
Flint and steel 1 per use
Fishing rod 1 for a fish, 2 with the bobber in a block, 3 with an entity hooked, 0 otherwise
Helmet, chestplate, leggings, boots the damage taken, before armour reduces it