Common mode signal appears equally (with respect to local circuit common) on both lines of a 2-wire cable not connected to earth, shield, or local common. Usually but not always, it is an unwanted signal that should be rejected by the receiving circuit. Common-mode voltage (VCM) is expressed mathematically as the average of the two signal voltages with respect to local ground

Normal mode signal is any type (other than common mode) that appears between a pair of wires, or on a single wire referenced to (or returned via) the earth, chassis, or shield. Normal-mode signals are read between two wires in a balanced or unbalanced transmission path. For a balanced 2-wire path, one wire is driven positive while the other is driven negative an equal amount, both with respect to a static or no-signal condition in which both lines assume the same voltage level relative to circuit common.
Differential mode signal appears differentially on a pair of wires in an ungrounded cable configuration. In some circles, differential mode noise has been given the title of "Normal Mode Noise".
 

Rated current is the level of continuous DC current that can be passed through bead or inductor.
For bead and ceramic inductor, this DC current level is based on a maximum temperature rise of the chip at the maximum rated ambient temperature.
The rated current is related to the chip's ability to minimize the power losses in the winding by having a low DC resistance. And it is also related to the chip's ability to dissipate this power lost in the winding. Thus, the rated current can be increased by reducing the DC resistance or increasing the inductor’s size.
For ferrite inductor, this DC current level is based on a maximum inductance change of its initial value.
 

The resistance of a bead or inductor measured with no alternating current. The DCR is most often minimized in the design of an inductor. The unit of measure is ohms, and it is usually specified as a maximum rating.

The impedance of a bead is the total resistance to the flow of current, including the AC and DC component. The DC component of the impedance is simply the DC resistance of the winding.
The AC component of the impedance includes the inductor reactance and AC loss.
The following formula calculates the inductive reactance of an ideal inductor (i.e., one with no losses) to a sinusoidal AC signal. 
                                                                                 Z = XL = 2πfL
L is in henries and f is in hertz.
This equation indicates that higher impedance levels are achieved by higher inductance values or at higher frequencies.
AC loss of the bead includes the magnetic hysteresis loss, the eddy current loss and dielectric loss.
 

Ferrite bead is a frequency dependent resistor and is used to help reduce unwanted noise. At low frequencies, inductive impedance is 10 ohm or less and the attenuation to the low frequency signal is limited. At higher frequencies, the impedance increases to over 100 ohms and becomes resistive above 100 MHz. The resistive loss attenuates noise at unwanted frequencies through heating of the bead’s ferrite material due to eddy currents.

Ferrites are a class of homogeneous ceramic materials composed of various oxides containing iron oxide, nickel oxide, zinc oxide, copper oxide, and so on. As a ceramic, ferrite is hard, inert, and free of organic constituents. Ferrite has many properties, which make it useful in electronics. It has a high magnetic permeability and therefore concentrates and reinforces the magnetic field. Ferrite has high electrical resistivity, which limits the amount of electric current in the ferrite itself. Thus, ferrites exhibit low energy losses and are efficient, and functional at many frequencies depending on the permeability. Ferrites can be manufactured in different shapes and sizes to address specific applications. In EMI applications ferrite absorbs the unwanted frequencies and releases the energy as small quantities of heat.

EMI is unintended high frequency electromagnetic radiation, which is generated in digital and analog systems. EMI can adversely affect the performance of a circuit internally and that of other associated equipment in close proximity or even far field.

EMC means the ability of equipment or system to function satisfactorily in an electromagnetic environment without introducing intolerable electromagnetic disturbance to anything in that environment.