A couple more examples are shown here. The first photo shows an aft lower fuselage panel removed. You can see the row of rivets running forward and aft just above the removed structure. Notice the skin overlap here is assembled over the lower skin. This prevents water from entering as it would if the lap was the other way around. Notice the step at this rivet row. This is a panel line.
The second photo you can see the seam where the fuselage structure is joined together. Look at the vertical line between the open panel and the paper taped to the fuselage. This is a production joint. These two sections of the fuselage could have been, and most likely were built in seperate fixtures and joined together later. This to is a panel line.
Basiclly the only way to not have panel lines on a plane is, the plane would require wrapping with one complete skin.
I hope this helps to define panel lines as we call them. In full scale sheetmetal talk we normally just say lap seams.