In general, boards can be categorized into three categories: rigid, flex, or metal-core.
Rigid boards are often the vast majority of boards a designer will encounter, where the layout of the board is contained within a rigid substrate created from a high heat and pressure lamination process. The common material for these boards is FR-4, but depending on the particular needs of the design, this can be modified to emphasize or otherwise improve certain characteristics of the board.
Flexible boards are composed of a less rigid material that allows for far greater deflection. The material is tactilely reminiscent of a film roll, and the board thickness is usually far less than a standard rigid board. While they already see major usage, there is hope that flexible boards will usher in the next step of wearable technology and remove the current planar constraints inherent to rigid board devices.
A metal-core PCB is something of an offshoot of rigid board designs, with an increased ability to dissipate heat throughout the board to protect sensitive circuitry. This style can be an option for high-current designs to prevent thermal wear and failure.
Wherever controlled electromagnetism exists, printed circuit boards form the infrastructure to maintain it. Of course, circuit boards don't just spring out from nothingness–their design and manufacturing are a huge engineering undertaking unto themselves.
