Let us consider a ferromagnetic material which is subjected to magnetizing field (H). The intensity of the magnetizing field (H) is gradually increased from 0. Then, the magnetic induction (B) also increases and becomes maximum (at point L in the graph). This state is called magnetic saturation.
On decreasing the value of magnetizing field (H), the magnetic induction (B) also decreases but the path that it follows is not the same. When 
magnetizing field (H) becomes 0, magnetic induction (B) doesn’t come down to 0 (the material is still magnetized). The value of magnetic induction (B) at which the magnetizing field (H) is equal to 0 is called retentivity or remanence of the material.
On decreasing magnetizing field (H) further from 0 (applying magnetizing field in opposite direction), magnetic induction (B) in the material decreases further and it becomes 0 (demagnetized) at certain value of magnetizing field (H). This value is called coercivity of the material.
On further decrement in the value of magnetizing field (H), the magnetization of the material is reversed and it saturates in the opposite direction (at point E in the graph).
Similarly, if the value of magnetizing field (H) is reduced to 0, the retentivity field appears in the material at point F. Again, on reversing the direction of the field, the material is completely demagnetized at point G (coercivity field). Then the material is again magnetized in positive direction and it once again gets saturated. This completes the cycle of alternating magnetization of the material and the curve repeats in other cycle.
In the process of alternating magnetization of the magnetic material, it is seen that magnetic induction (B) lags behind magnetizing field (H). This phenomenon in which the magnetic induction (B) lags behind magnetizing field (H) is called magnetic hysteresis. The B-H curve for one complete cycle is called hysteresis loop of the material. The area covered by the hysteresis loop represents the loss of energy within the material when it is magnetized and demagnetized. The loss of energy is called hysteresis loss and it is in the form of heat energy.
