The multilayer technology essentially introduces a vertical solution approach to the problem of the conflicting requests for extremely high number of interconnections and reduced sizes in a single printed circuit board.

The interconnections among the active and passive components that are (typically) mounted on the top and bottom of the printed circuit board are realised in some internal layers that are pressed together and that can be then connected the one to the others (through blind and buried vias) and to the superficial (external) layers of the board (drilling through all vias).

Basically, each internal layer of a multilayer printed circuit board is essentially a double sided pcb, but with no solder and finishes, and its manufacturing process requires the same steps of a standard double sided pcb, with the only exceptions mentioned above.

The more the internal layers are the more difficult becomes the manufacturing of the printed circuit board but, on the other side, the higher the interconnections density available and the performances.

Multilayer pcbs are typically classified in standard and high tech boards. They are called high tech multilayer boards when they consist of more than a certain number of layers (basically 12 layers and in this case they are also called high layer count boards) and when they require more than a single pressing step (sequential lamination and sequential build up technologies) or laser drilling (microvia) technology.

Without the multilayer technology no miniaturisation of the printed circuit boards would have been available when combined with the increasing number and complexity of the active and passive components mounted on them.
In applications like mobile phones, for example, where the size of each element of the device (boards, antennas, chips, etc) has become more and more critical, only the introduction of the multilayer technology in the printed circuit boards manufacturing allowed the producers to match the market requests.