Iron is a metal resource from vanilla Minecraft, and an ingredient in nearly every recipe in IC² in one form or another.
These forms of iron are seen during ore processing:
These forms of iron are seen during metal shaping:
Pretty much everything ever.
No, seriously. The first thing you'll need iron for is crafting the Forge Hammer to make metal plates, in order to craft an Iron Furnace or Basic Machine Casing for your first Generator - there are very few things that don't use iron in IC², and most of them are subcomponents of things that do! Or maybe they're steel instead, which is just an upgrade of iron anyway.
You're going to need lots of this stuff. We mean it.
Basic crafting is the same for (almost) every metal. Any pre-ingot form, except for the Tiny Pile of Iron Dust, can be smelted into an ingot.
Smelting anything but dust is a waste of resources, though - check the ore processing page to see how to maximize your output.
9 ingots craft into a block and vice versa.
Dust comes from 9 small piles, but you can't craft small piles out of a single dust like you can craft ingots out of a block.
For parts that you make using this metal, plates and item casings and so forth, there's a page for that already.