Unbreaking is an enchantment that is applied to tools and weapons to avoid durability reduction when used. This helps increase the lifespan of the tool. The Unbreaking enchantment is mostly used with tools such as Axes, Pickaxes, and Shovels.
Mining copper with a pickaxe enchanted with unbreaking (via screenshot)
The three different levels of Unbreaking all provide a chance for the tool being used to avoid losing durability when used. On average the tool will last enchantment level + 1 times longer than normal. For armor, there is a {60 + 40/(enchantment level +1) } chance that a single use of the item will not reduce durability. To simplify this, Unbreaking armor has a 20%, 27%, and 30% chance of ignoring durability loss.
There are multiple ways to enchant tools with Unbreaking. The most common and reliable way is through an enchantment table. Players must have the Lapis Lazuli as well as the experience required to get it.
Getting Unbreaking III from an Enchantment table (via screenshot)
Secondly, you can trade with Librarian Villagers that can be found in villages. Other ways that are less reliable but still can be used consist of fishing and loot chests. If you do manage to get your hands on an enchanted book with Unbreaking, you can use an anvil to apply it to the tools.
Applying Unbreaking III to a pickaxe with an anvil (via screenshot)
Unbreaking is an excellent enchantment for people who like to farm for resources such as iron, gold, etc. It is widely used as its durability negating effect is quite helpful, since tools can break faster than you realize sometimes. Since it can be easily obtained, it is definitely recommended that you make use of it.
A. It doesn't last forever but on average it will last around four times longer than regular.
A. The highest level of Unbreaking is level III (3).
A. No, Mending is definitely better than Unbreaking.