A PCB is defined by the designer with a number of copper layers in a well define sequence. The pattern in each copper layer combined with the drill patterns and the copper plating of these drilled holes, form an electrical net that is responsible for connecting the pins of components to other pins of other components. This makes it possible that currents can flow and the application can function.
The sequence in which these copper layers are defined is called the Board BUILDUP.
The picture shows a 6-layer board presented in our buildup wizard which is a part of the Eurocircuits calculator. Read also.