Difference Between Iron and Steel

See Previous Metallurgy Posts!
APS 19.10 Metallurgy – Man and Metal! (copper)
APS 19.11 Metallurgy – Man loves Heavy Metal! (bronze and iron)
APS 19.12 Metallurgy – Viking Technology (steel)
APS 19.13 Metallurgy – Man Loves This Metal (gold)
APS 19.14 Metallurgy – Man Loves This Metal Too (silver)