Iron ingots can be primarily obtained by Smelting (with the help of a Furnace) of iron ore found while mining. However, they can also be found in Temple, dungeon, and stronghold Chests. 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 amount of crafting recipes which are about 29 total.
Because it's a rare drop by zombies, iron ingots are considered a renewable resource.