Why do iron and steel become magnetized?

Iron and steel, the latter of which is an alloy of iron, carbon, and perhaps a few other metals, are sensitive to magnetic fields. In addition, the atoms are able to reorient themselves within the metallic crystal lattice to align themselves with the magnetic field. This creates what are termed magnetic domains. In summary, the magnetic domains created when the specified materials are exposed to a magnetic field will retain an "imprint" of the field that created them. The iron or steel is then said to be magnetized.