Ferromagnets below T_c have magnetic domains: regions of uniform magnetization separated by domain walls. An applied field H causes domain walls to move, switching domains parallel to H. The hysteresis loop (M vs H) arises because domain switching is irreversible — it requires overcoming pinning energy barriers. Key parameters: saturation M_s (all domains aligned), remanence M_r (magnetization at H=0 after saturation), coercivity H_c (field to demagnetize). Loop area = energy dissipated per cycle.