Iron Ingots can be primarily obtained by the Smelting (with the help of a Furnace) of Iron Ore found while mining. However, they can also be found in Temple, Dungeon and StrongholdChests. Rarely, Zombies will drop items made of Iron, due to them wearing Iron Armor or holding an Iron Tool, or just the ingot itself. Iron Golems drop 3 to 5 iron ingots when they die.
Iron is one of the most useful Items in-game, as it creates tools that are capable of mining all Ores and anything but Obsidian (which can only be mined with a DiamondPickaxe). It also creates the middle-tier armor, with Diamond being the highest tier. This makes it perhaps one of the most-used materials in Minecraft.
Iron is used in the most crafting recipes: 29.
Because it's a rare drop by zombies, Iron can be considered a semi-renewable resource in Minecraft.