Iron Ore is a vanilla block naturally generated in layers 1-64. There are about 77 iron ore blocks per chunk (in vanilla minecraft). Iron Ore can be aquired through the use of a stone pickaxe or higher.
Iron ore veins can vary in size, though the most common size is 2x2x2. Each block in the vein which would otherwise be stone has a random chance of being iron ore instead (therefore it is good to check around a located ore block for more ore, as ore blocks may be only diagonally adjacent rather than face-to-face). If the vein intersects a cave or other structure, part of it may be replaced with empty air. King Lemming's Thermal Expansion increases the size of all vanilla minecraft ore veins in the 1.5 packs.