In PCB design, what is the common-mode current?

In PCB design, common-mode and differential-mode current are refered frequently. But I don't understand the meaning of them and what condition they will be produced at and how they be produced. Anyone could explain them for me?
P.S. Attached file is the figure of common-mode and differential-mode current in book.

Transmission lines are designed and advertised with a certain characteristic impedance value. This impedance is established by the ratio of distributed inductance and capacitance in the transmission line. This impedance, if applied as a resistance across the line at the far end, would make voltage and current uniform along the line (except for the small normal power loss with distance). Because this impedance relates directly to opposite flowing currents in each conductor, it is called the differential mode. It deals with the “across” or between conductors characteristics.


A conventional two-conductor transmission line, even if one conductor is called the “shield”, must have exactly equal and opposite flowing currents into each conductor at each end. Without equal and opposite (differential mode) currents flowing at every point in a transmission line, it will radiate and receive signals. A transmission line with purely differential mode operation would never radiate unwanted energy. It also would not respond to outside radiation or signals.




Common Mode Current
Transmission lines may have another mode, conducting currents known as parallel or common mode currents. Common mode current is the portion of conductor currents not matched by exactly opposite and equal magnitude currents. This is the portion of total current responsible for a feedline behaving like a single wire line. Common mode current is most commonly caused by improper feedline installation or antenna design. The feedline, in effect, behaves like two very different transmission lines connected to the antenna and equipment at the same time.


As with differential mode’s impedance across the feedline conductors, parallel or common mode operation has impedance to “ground”, to other objects around the feedline, and to other points in the system. It is often useful to consider this the system impedance when fed like a longwire. Common mode voltage differences along the line cause current to flow, and the common mode impedance determines current flowing in that mode. The voltage that causes common mode current almost always appears at the antenna, since that is where major balanced to unbalanced transitions occur.

Thank you for your answer. So, the source of the common-mode current is the source of RF transmitter,isn't it? The RF is received and return to the Earth by our system. Is it right?